I just used Simple Green to clean some denim jeans that were covered in grease from my car's suspension. I put the jeans and about 8 - 10 ounces of Simple Green in a 5 gallon bucket of water. I let them soak for about an hour, agitating by hand a few times. The water was so dirty that I emptied the bucket and repeated the process once more. After rinsing, they looked pretty good, but they smelled a bit funky, so I ran them through a regular machine wash cycle. Came out perfectly clean!
On tv, they said to use a degreasing dish soap like Dawn, that they use to remove oil from wildlife. Soak it in Dawn, rinse it out, put it in the washer with just a SMALL amount of Dawn so that you don't get too many suds. Then you might want to re-wash normally before you dry it. It should remove the oil.
I have no experience doing this, but here's how I would approach it.
First, remove as much oil as you can. Use something like paper towels to soak up the oil on the surface.
Next, get a bucket large enough to fit the backpack. If you can, turn the backpack inside out. Fill the bucket with hot water and immerse the backpack. Skim or pour off the oil as it rises to the top. I'd let it sit in the hot water for a while, maybe an hour.
Then take the backpack out and scrub it with a stiff brush and a good surfactant. Rinse it well, and hope for the best!
When I got engine oil in my back pack, I sprayed it down with Gunk engine degreaser and used a scrub brush and hosed it down. I've seen similar products with a bioremediarion work even better and quicker.
You could use the same technique that I devised for removing clogged fuzz from Velcro: add fish oil and throw it into a room full of cats.
Actually, the larger question here is why the hell you had a bottle of olive oil in a backpack. Have massage table; will travel?
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