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.