ethanol in gas bad for our cars?
I think he's probably wrong.
Re: the leave-the-cap-off-the-gas-can theory.
I can promise you he's wrong. There are several gasoline components more volatile than ethanol, particularly in winter blend fuels. Ethanol boils at 78 Centigrade, some gasoline components boil as low as 0 Centigrade. Those volatile components are there to help with cold starts. That's most of what that mechanic is evaporating from his fuel
Leaving the lid off the can also invites the fuel to absorb water from the air, which should be avoided. It can lead to phase separation, where a water/ethanol-rich layer settles to the bottom of the gas can. Not good.
As for Marvel Mystery Oil, I have used it successfully as a fuel additive to clean a fuel system. However, the MMO formulation greatly pre-dates the use of ethanol in gasoline, so that particular additive was never formulated to deal with ethanol-related problems.
