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Just take some of it along with your waste oil, or mix it in, etc.
DO NOT DO THIS!!
Gasoline will mix with used motor oil just fine, BUT the vapors will still come up into the top of the tank and will be extremely flammable.
When somebody comes to pump the tank, be it Safety Kleen, or any others, they usually check the vapors for flammability, the whole tank could fail that test and be rejected.
It will also put others in danger of an explosion. At my local O'rielly's that tank is in the back of the store, near the double doors for access. Guess what else workers do out there? Smoke.
Recycling of used oil is a great benefit to me and is usually a free service. Don't do anything to screw this up.