Hi Walter,
The OS 40 FP was a economy engine that many flyers grew up on . Its a ABC (Aluminum,Brass,Chrome) type with out bearings. We used to say--FP meant Front Plain . After a day of flying ,I always take out the glow plug ,and put some after run oil in the motor. Looks like Automatic transmission oil (ATF).Put the whole front of the plane in a freezer bag ,or large plastic bag,with a wing hold down rubber band to keep it attached. Catches the oil that seeps out. Will also keep your mother happy.
If your engine is really gummed up--Find a old used crock pot . Fill it half full with car antifreeze (50/50 mix-1/2 half antifreeze-1/2 water).Put your motor in it ,turn on -plug in the crock pot ,and slow cook your motor for about 8 hours.This will dissolve all the gum,rust ,and make the motor look like brand new. COMPLETELY take the whole motor apart ,and wash in dish soap water,rinse,dry,then oil the motor with WD-40,or any good lite weight oil. Reassemble the motor,put in a new glow plug ,fresh fuel.I would bench run the motor. allow it to get nice and warm . Try to avoid running it lean. Peak the motor out -Hi speed ,then richen it by 1/4 turn of the needle valve. reinstall it in the plane ,and it should be fine--good luck--hope this helps you out some ......Amelia