I am not sure who came up with the WD-40 is bad for engines but here it is off the can first thing listed.

Lubricates-wheels,rollers,hinges, chains and gears.
cleans-grease,grime ,tar,adhesives,gum.
protects against rust-tools,firearms,spoting equip.
displaces moisture-to restore wet or flooded equipment such as;engines,spark plugs,power tools.

isn't that what you want in a lubricant and a protectant.
I don't use it for long term storage but for short term it has worked for me quite well I happen to live in the maximum rust area,4 blocks from the beach and my engines run fine every time I start them up and there not rusty.
I also use air tool oil as well as most all of the afore mentioned oils for my work.
I would like to here from the lubrication experts out there exactly why not to use WD-40, mabee I will stop if there is some compelling evedence why I shouldn't,BTW if you have rust in your motor clean it out before running it,it will not matter what oil you use the motor will be toast after you start it and all the rust particles score the piston and internal parts.
as for Marval Mystery oil if you go to the gas engines forum there are pages and pages of recomendations on not to use it for your engines