Explosion....
Mirage is completely correct in this, as I didn't bring it to your attention. But I did only recommend that you use it on your oil tank. I did say, that the oil mix in the gas tank will not hurt anything to continue to run. And if the amount of gas you put in the oil tank is minimal, it won't hurt either. It will lower the viscosity of the oil's properties, but with the premix in the fuel tank, it shouldn't matter.
This is all based on whether you put a cup or so in, or a gallon. The flash point of gasoline is 90*F or less. Which makes the volatile gases explosive. Mixing with the oil, increases the flash point dramatically and lessens the chances of explosion. Then, the amount of gases has to concentrate in high enough quantites before an explosion can occur. So, if you did, do not suck in just the liquid, leave your self open to the atmosphere also, for adequate ventilation.
I only use my shop vac to suck oil from my engine. That's all I use it for. It's very old and I bought a new one to replace it. This works really well for me instead of the high priced pumps. But to be on the safe side, like Mirage says (and thanks for pointing that out), you can build explosive gases inside the shop vac, where it is exposed to the top of the motor's brushes, where there is a source for spark. So, I would not advise using a shop vac on any type fuel tank. Which again, I did not recommend it's use in that application.
If you put a gallon or so of oil in the gas tank, simply siphon off what you can and fill it back up with gas. Either way, a little extra oil shouldn't hurt anything. But if you have gas in your oil, you'll be almost required to "premix" until you are sure there is no more gas in that oil tank.
