Would diesel be a good engine cleaner / corrosion inhibitor to spray on the motors to keep them looking good?? I know WD40 is bad as it swells the wiring insulation - so are there any reasons not to use diesel??

no! once it gets hot it ignites like gas..you need to look into fluid film,,its pricy but its one of the best out their..

http://www.fluid-film.com/

BTW you can use it on anything! as a wax and wipe it down..on your outdrive just spray it on and wipe it off and it looks like you took a buffer to it..i buy it by the 5 gallon bucket for the salt trucks in the winter time..
look at the pics at the bottom of the page..
http://www.theruststore.com/Fluid-Fi...l-P182C67.aspx

another down side to diesel even if boat doesn't burn to the waterline...it smells bad!!!

That's a very good point- so obvious I didn't think of it. In my defense I don't think it's hot enough in the engine compartment to ignite diesel as you really need to hold a open flame to it before it'll light. WD-40 burns too.... hold a lighter up and spray the can into it . Thanks for the Fluid-Film link it looks like really good stuff.

i use marvel mystery oil,that chit really works good,i put it in a spray bottle,spray it on,then wipe the excess of with a paper towel.