RE: WD-40 for cleaning engine?
I use WD-40 in the same manner as your friend does.........works great and doesn't get gummy like castor after-run oil does if left for long periods. I do not, however, leave fuel in the tank while in storage. I actually use a ton of WD-40 on my models! I used to clean the model with alcohol but that gets expensive. Now my cleaning steps are air nozzle, 409 w/brush, rinse w/H2O, dry w/air nozzle, spray all metal with silicone, and finally spray all bearings and carb with WD-40. I then put the drill starter on it for a couple turns, wipe it down with a cloth, and put it up.
When I fire the engine it generally take about 20 seconds before the WD-40 cleans out and normal engine operation resumes. WD-40 is simply the best low cost lube for bearings. The only thing that beats it is TRI-FLOW lube which is expensive and really not needed.