Gasoline is less dense than oil, so...

Jul 13, 2020
Wouldn't upper cylinder lube like Lucas or MMO eventually live at the bottom of the gas tank? I once added MMO to a half-empty tank, then topped off. My car immediately started running very rich with fuel trims in the high negative 20's. It evened out shortly afterwards, but that was enough to give me pause. I'm guessing there was a glut of MMO right near the inlet of the fuel pump. Also, if you use this stuff regularly, wouldn't it continue building up in the bottom of the tank and either never get burned, or eventually reach the fuel pump and make bad things happen? Thoughts?
Lucas and MMO should mix with gasoline and not separate. Both, gasoline and oil are nonpolar and miscible. If you mix a polar liquid with a nonpolar liquid the more dense liquid will settle at the bottom, as is the case with gasoline and water.
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As mentioned above, they are miscible and mix together even though they may have different densities. Ethanol is less dense than water also but the two bond together based on their common hydrogen molecules.

To answer the question - No, the upper cylinder lubricant and Marvel Mystery Oil won't separate and settle to the bottom of the tank.
Ah yes, I now see that my hypothesis is incorrect. Kind of silly now that I think about-I'm sure someone would have noticed this before I did. I must have a leaky fuel injector or something. I also recently had a misfire at startup. Although, is it still possible to have gotten a large concentration of MMO briefly, maybe before it was mixed in thoroughly?
What is it exactly in the UC that isn’t getting L and needs to be added to the fuel?
Probably nothing. I had some MMO on hand and was curious as to how it would affect my rough idle and wacky fuel trims.