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At what point has the oil gone bad?

  1. Jul 31, 2015 #1
    I'm now driving a '72 VW Beetle and by all accounts the oil must be changed in this car much more often than in cars with other types of engine. This engine has no oil filter. There is a coarse strainer, but particles would have to be quite big for this to come into play.

    I have been experimenting with changing the oil at shorter and shorter intervals in the hope of squeezing as much life out of the very tired old engine as I can.

    Here is one recommendation I found somewhere on the interweb (can't recall where):

    Rather than estimating by mileage and conditions, I'm wondering if there is a way to simply examine the oil and know if it is time to change it. I am currently changing it once a month, but even on this short schedule, the oil that comes out is quite black.

    The oil seems to be quite clear for about a week, then it starts to take on a tea-like brown-ness that slowly progresses over the next three weeks to total blackness.

    Does total blackness indicate it is no good anymore? Does it mean the oil has pretty much "broken down," or is there life left in it? Is there any way to physically test the oil?

    Thanks in advance.
  2. jcsd
  3. Jul 31, 2015 #2
    Yes there are test kits for oil Google "engine oil test kits"
  4. Jul 31, 2015 #3


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    If the '72 engine has never been rebuilt with average mileage for that age (+ 200K?), then the rings alone are probably letting a massive amount of fuel and exhaust into the oil.
  5. Jul 31, 2015 #4

    It looks like most of the kits are things you have to send to a lab for analysis. That's not what I'm looking for.

    However, there does seem to be a device that instantly reads the state of your oil.


    This might be worth it if it actually works as claimed. Thanks for the suggestion.
  6. Jul 31, 2015 #5


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    Quick DIY tests are :

    Filter a sample and examine sludge - tea strainer and loo paper will do .

    Dip magnet into sample and examine what gets picked up .
  7. Jul 31, 2015 #6
    I don't know the engine's repair history at all, as the guy I bought it from didn't know. It says 30,000 miles. I don't know if that's 130,000, 230,000 or 330,000. It seems to be an original engine number from '72, but it could have been swapped from another '72 or taken apart at some point and rebuilt. At any rate, it does not burn oil that I can see. The exhaust is clear and neutral smelling. The guy who sold it to me did claim he was told it had been stored for 13 years at some point. I found an old repair manual in it in which someone had made a lot of notes. That owner was obviously serious about maintaining it. I also found a piece of stationary or note paper stuffed in the manual from an auto battery shop in Tampa Florida on which the guy had written some notes about adjusting the carb. So, I think it was originally sold there in Florida and then driven here. In other words, there's a good chance it hasn't been stressed out by cold weather operation.
  8. Jul 31, 2015 #7


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    Duh on my part. If stuff is getting past the rings, it would go both ways. :rolleyes:

    So, no burning oil, unknown miles. Oil that gets black quickly could indicate overheating but I'll assume that an air-cooled engine tends to run hotter. Is the oil getting dark faster recently or is it just that you've started paying more attention to how fast it gets black and this is the pattern that you're seeing?
  9. Jul 31, 2015 #8


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    Also, Nidum's test is a good one. If the oil has metal in in, it definitely needs to be changed.

    Something else to check. Pull out the dipstick and examine it. As old as it is, it probably has a brown varnish on it after cleaning (unless it's been replaced). Is the varnish light or dark brown? That would give you an indication as to the varnish buildup inside the engine. A darker color would mean more buildup which may be soaking into the oil as it runs.
  10. Jul 31, 2015 #9
    The lore about bugs is that the oil has to be changed much more often due to the lack of an oil filter. An oil filter would obviously be filtering metal particles out, but I'm wondering if it is also alleged to filter out anything else. Does it keep the oil from becoming black for a longer period of time? I'm not sure. I don't believe the blackening represents a build up of metal particles. I have the idea it represents a chemical change in the oil, possibly due to pollution from contaminants.

    That said, I do get some light ferro-oil from very finely divided metal particles on the surface of the metal plate that holds the strainer in place. I can't tell if that's fresh each time or if it is being washed there from other parts of the engine where it may have gathered over the years. Likewise, I am not sure how much of the black color may be very old sludge that built up in the past when some previous owner didn't change the oil often enough. That is why I'd like to be able to test the oil, itself, and be able to change it when it actually goes bad, rather than according to a schedule that might not be keeping up with how fast it actually degrades.

    Yes, I believe the air cooled engine always runs hotter than a water cooled one. This might be the actual reason you're supposed to change the oil more often, I'm not sure. Is that true of motorcycles, which are air cooled? Change the oil more often? In motorcycles the air cooling is passive. In Beetles, it's force fed over the cooling fins by a fan, so, even if you're idling at a red light, it's being cooled. In any event, no, it has not been getting darker faster. It has held steady since I got it in December. I have been changing the oil at shorter intervals, though, shifting from every 3000 miles to every 4 weeks, regardless of mileage.

    The question I am asking, I guess, is if I should be changing it before it ever gets completely black? Does completely black always mean it's over-the-hill?
  11. Jul 31, 2015 #10


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    I don't see the problem. Older cars pretty much had black oil with every change. Newer cars can be still be close to the original color.

    Unburnt carbon can collect on the cylinder wall with each compression stroke. The job of the oil is to lubricate and clean the cylinder wall which it seems to be doing. A filter would have been a good thing. The oil also has the function of cooling engine parts, so a right good reason to not let the level drop too too low.

    Short trips don't give the engine and the oil enough time to heat up, and with that the volatiles from the combustion will enter into the oil and not dissipate. You might notice some grey sludge or gunk around the filler cap if you have been running the engine too cold.

    I don't see any reason than to follow the manual as to type of oil and frequency of oil change, per type of driving that you do, and the outdoor temperature. Changing oil earlier than recommended is just a waste of time and money, but if it gives you piece of mind then go for it but not to extremely. Oil just does not lubricate one day and the next it doesn't. Manufacturer's recommended oil changes are probably a compromise between the goodness of the aged oil, wear of the engine, estimated life of the car itself, and cost of the oil change. Are you going to get postive economic returns from changing the oil more often, especially at around 40 bucks a pop.

    You would know if your engine is using oil by checking the dipstick. If it hasn't dropped by much ( most older engines did actually use a bit of oil ) from the oil change to right before your change. I would be more worried if the oil level is dropping and the oil is not providing adeqauate cooling form the bearings, valves and other moving parts.
  12. Jul 31, 2015 #11
    If the black color is just carbon scraped from the cylinder walls, then that's fine, and I won't worry about it.

    It actually only costs me About $12 per change. The crankcase will only hold 2 1/2 quarts, and I buy the cheap, AutoZone brand at $4 a quart. Do the labor myself: takes a half hour.

    Something odd about the old VW engines is that they never incorporated any temperature sensors. I bought an aftermarket oil temperature sensor that replaces the dip stick. If the oil gets above 230F a light will come on at the dash. So far, this has never happened, and when I physically look at the sensor it's clear it's not even close to the critical point. This might indicate I rarely do get up to proper operating temp, and the blackness in the oil is the unburnt carbon you suggest.
  13. Jul 31, 2015 #12


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    Meet any Susan Pleshette looking admirers yet.:biggrin::cool:

    I think the interior heater and windshield defrost for cold weather driving is kind of funcky also.
    It was/is kind of lean in some areas.
    Can't remember if the car came with a spare tire and jack.

    I wonder what an exhaust emission test on the Bug would show, and how it would compare to other vehicles of that era, just out of curiosity of an air-cooled vs water jacket cooled block. There has to be some differrence between the two types as the motor is put through its paces.

    I'll have to try to look that up.
  14. Jul 31, 2015 #13

    jim hardy

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    You are fortunate ! No computer, no oxygen sensor, no problems!

    Old engines get sludge - the black goop settles on engine's internal surfaces. That your oil takes a week to turn brown says your engine is remarkably clean.

    There exist detergent additives that attack sludge. They should not be left in more than about a half hour lest they free up enough sludge to plug the strainer as happened to my neighbor. It might be worth trying one , but do it on oil change day and run the engine only a few minutes with that stuff in there and drain it immediately.

    API oil ratings keep changing. They're up to SJ now.
    The S means it's for Spark ignition engines, ie gasoline.
    C rated is for Compression ignition(diesel) .
    J means it's the tenth revision to the standard.
    EPA is making refiners take out the zinc and phosphorous that help lubricate because they are hard on oxygen sensors.
    So i advise don't buy cheap modern oil. It may wreck your camshaft .

    I use the API C-rated oil Shell Rotella or Chevron Delo, even Walmart's house brand when it's on sale. C means it's suitable for diesel engines so i figure it's overkill for gasoline.
    Aeroshell W is made for air cooled engines, you can get it at airports or motorcycle shops.

    Here's a blurb on oil additives and camshafts from folks at Land Rover .
    so your cheap oil may be "worn out" before it even gets put in the bottle.

    hmmmm i wonder if that 'sludge' has ZDDP in it ? Maybe you want to leave it alone.
  15. Aug 1, 2015 #14

    Ranger Mike

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    Ifin it aint broke don’t monkey with it.

    I have found that you will not be able to filter fine enough to drop wear particles that you can afford to test for. Spectrographic analysis looks at 10 mic and smaller for wear rates. That includes bypass filtration on the vehicle. Let’s look at the practical side. For a daily driver trapping 20-25microns and getting a filter that doesn't go into bypass mode every time the temp is cool or when it pressure surges is very important. Even race filters use a heavy paper media and is efficient to only 98% at 35 microns,50% at 16 microns. So the amount of 10-20 microns particles trapped is not that impressive.

    The dirty little secret ( no pun) is most wear particles that do any real damage are 20+ microns or larger. That’s 0.0008”. One thing people forget is that EVERY automobile engine has a bypass mode and unfiltered oil can by-pass the filter if too cold or pressure surges. The venerable small block Chevy engine never filtered oil on the rear most main cap and unfiltered oil could flush thru the number 5 main bearing every revolution. This by pass mode may be more critical to reduced engine wear than the ability to remove fine particles. The smaller debris can be successfully and efficiently dispersed in a high quality lubricant. Don’t forget you are floating the rods and main bearing on a film of oil. The typical crank shaft main bearing clearance is 0.0025” or 62 Micron so a 25 micron chunk can blow thru pretty easy without impact on the bearing surfaces.

    The Germans spent a lot of time and money on the old VW oil system. So do what the factory tells you.

    I would do a compression test on the cylinders to see what the rings are doing and if all readings are within 20% ..run it. A leak down test is more complicated and tells about valve sealing but those can be popped on without pulling the engine.
    Just remember the OIL is cooling as well as lubricating in the VW engine so fluid levels are critical.

    One more thing..i hate the fact I gotta change oil every year in the dodge diesel Cummins tow vehicle. Because fuel and ignition chemically changes the oil after a period of time, you have to spend the money or suffer serious $$$ due to lack of proper lubrication. Same with Otto cycle engine oil. After time, the chemical composition changes and you got junk oil.

    Remember the air filter is more critical to clean engines than the oil filter. You've already sand blasted the top end of your engine if the air filter is damaged. Then the crud migrated to the oil pan.

    If you are racing using a heavier oil filter case and purely synthetic media … nuff said?
  16. Aug 1, 2015 #15
    OK, I haven't run into this info about the zinc and phosphorous before.

    You say the cheap modern oil might wreck my camshaft, but wouldn't the expensive modern spark ignition oil also have no zinc and phosphorous?

    It sounds like the only oil that will have the necessary zinc and phosphorous is diesel. That's what you're saying, right? I'm pretty much limited to C rated oil?
  17. Aug 1, 2015 #16
    Thanks, Ranger Mike!

    The Beetle guys do emphasize that it's an air and oil cooled engine, so I have been pretty attentive about the oil level, checking it at least every other day to make sure it's not getting low.

    My Bug has the original oil-bath air filter. This is a hilarious device filled with coconut fibers that are wet with oil to make them sticky. The air is forced through a chamber stuffed with this on its way to the carb. You never change the fiber, you just dump the oil that sits in a shallow layer at the bottom once in a while and replace it with fresh oil. I get such a kick out of this: coconut fiber! It's so 1930's, but they were still doing it in the 1970's. Some people get rid of this and replace it with a paper filter of some sort, but I wouldn't part with this original system for anything.

    Speaking of metal particles, I decided yesterday to put a magnet on my oil drain plug in the hope it will arrest them right there. I've got a good strong rare earth magnet on it now. Just such a magnet is built into the transmission drain plug, and I don't know why they didn't do this to the oil drain plug as well.
  18. Aug 1, 2015 #17

    jim hardy

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    Since diesels now have emission controls i assume EPA has taken the ZDDP out of C oil too.
    I spent some time on API and Chevron and Shell's sites. Shell assures their Rotella has something that assures it meets all prior API wear tests. So does Chevron.
    Probably so does their S oil but i was looking specifically at C oil.

    My reasoning for C oil is this, and it's not mainstream:
    with their twenty-something to one compression diesels really hammer crankshaft bearings. (That's why GM's small block diesels were such a catastrophe for them, they broke the main bearing supports..)
    Any oil designed for that service is stout.
    It's not what is recommended in my owner's manual. But nothing i own is still under warranty.

    So i use C oil on a 7500 mile change interval.
    At oil change facilities I have to sign a waiver relieving them of liability for C oil in a S engine, which i gladly do.

    Read the fine print on the back of your cheap oil bottle.
    Does it say "Certified to meet API SJ"
    or does it say "Recommended for API SJ" ?
    Spend a few minutes reading oil bottle labels...

    Look at the back of Walmart's real el-cheapo bargain oil - it says "...not suitable for engines manufactured after 1931".

    So, i've used C oil ,at my own peril,
    in my Oldsmobile for >ten years 160k miles,
    in a Dodge Minivan (Mopar 3.0v6) for 180 Kmiles(from 130K when i got it to 310K when gave it to a neighborhood youth),
    and several other vehicles along the way.
    It's gone from $8 a gallon to $13 but there's no inflation.

    Maybe a genuine mechanical engineer will chime in with some facts - all i have is my observations.

    anyhow - buy good oil.

    old jim
    Last edited: Aug 1, 2015
  19. Aug 1, 2015 #18
    I decided to do some googling, and this site says the whole thing is moot when you have a stock engine:


    The AutoZone cheap oil does not employ the terms "certified." It boldly asserts, "Meets or exceeds warranty requirements for today's gasoline engines where SAE20W-50 motor oil is recommended." That's on the back. On the front it says, "Meets API performance standards."

    At any rate, I happen to know for sure the previous owner was running either straight 30W oil, or 20W-50 from 2006 to Dec 2014, depending on if he changed the oil himself or if his mechanic did it. Neither of those is the old high zinc oil, which started to be phased out in 1996, according to the omniscience of the interweb. I believe any damage that might occur to the camshaft must already have been done.

    Your reasoning for using the diesel seems sound to me. I might actually try it. It sure doesn't sound expensive.
  20. Aug 1, 2015 #19

    jim hardy

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    This link has more than any reasonable person could want to know.


    this from page 35 is cryptic enough to worry me

    What is a non-petroleum engineer mortal do ?
    All i know to do is the best i can.... buy good name brand oil, pay attention to how the engine sounds.

    If that VW doesn't have an oil pressure gage you might consider installing an aftermarket one.
    But plumb it with metal tubing, or get an electronic one.
    You don't want to pump your oil out through a leak you've made.
  21. Aug 1, 2015 #20
    Oh, it has an oil pressure gage. That came standard. The thing VW's didn't have was any kind of engine temperature sensor. They didn't include one for some odd reason.

    You have two warning lights. One is the oil pressure warning light. The other is the generator/alternator(depending which you have) warning light. If the latter comes on it usually means the generator/fan belt has snapped. That's the only belt on the engine: it runs the generator off the crankshaft. The cooling fan is attached to the forward end of the generator shaft, so if that belt snaps, you lose both electrical and cooling. That warning light might also merely indicate a connection has come loose from the generator, though. Anyway, if either light comes on, it means pull over ASAP.

    My oil pressure warning light came on only once, so far. It turned out the sensor needed to be screwed in more tightly is all. In some cases it means the oil pressure relief valves are sticking due to being dirty; an easy fix.

    Both lights come on when you are starting the engine to tell you they are working, then they go out once you release the ignition key. I always watch for them like a hawk.
  22. Aug 2, 2015 #21
    I don't get this reference.
    Yes, the heating system is all different because it's air cooled instead of using a liquid coolant. The air used to heat the car's interior comes through a heat exchanger built around the exhaust pipes. If there's any breach of the exhaust pipes, obviously, someone could get killed. Hehe. German engineering. Someone who owned my Bug before me was so scared of this that they plugged the heat tubes with wads of paper towels. Even if someone pulled the vent opening levers by accident, no heat (or CO) would come into the cabin.

    The other fun thing they did was put the battery under the rear seat. Nothing wrong with that in principle, but the rear seat has metal springs that often made contact with the battery terminals when sat on and started the seats on fire. To be fair, theres a little plastic guard you're supposed to put over the positive terminal to prevent this, but why didn't they just do something about those metal springs?

    Amenities are sparse in the Air-Cooled Beetle, but they did come complete with a spare and jack, and also a dedicated tool kit: a wrench specific to the oil drain plug, one for the transmission oil drain plug, one for the fan belt pulley, etc.
    I don't know how it compared. There is a famous VW Beetle mechanic named Hoover who bemoans the fact that no remaining Beetle runs the way they did when they were factory fresh: no one knows how to service them anymore, and people put all kinds of wrong parts on them, and newcomers are copying these bad practices without knowing they're wrong. An emissions test on a currently running Beetle wouldn't tell you what they were actually capable of when new. There was a big movement in the 90's to replace the vacuum advance distributors with centrifugal ones. These require a whole different timing, but some people don't even know that, and drive them around running like poop.
  23. Aug 2, 2015 #22
    I went to AutoZone today and looked at the diesel oil. They carry all the ones you mention, and they are very inexpensive. I didn't buy any at this point, but I got a bottle of zinc additive for $4.00. It should be good for two oil changes (one bottle per 5 quarts of oil, and I only can fit 2 1/2 quarts in my crankcase), but I might actually use a whole bottle since it's a small bottle (maybe 1/3 quart) and not expensive.
  24. Aug 2, 2015 #23


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    Some VW engines had components made from Magnesium alloy . Just a hunch but it is possible that some modern oils with complex additives will react chemically .
  25. Aug 2, 2015 #24
    Yes, the case was an aluminum/magnesium alloy. However, I haven't run across any complaints or rumors that these are attacked by modern oils. There is a large faction of people who claim ethanol gas additives are damaging things, though. I'm not sure how much proof they have.
  26. Aug 2, 2015 #25

    jim hardy

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    Thanaks - i'll look for that stuff !
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