For small motors in general I use non detergent motor oil.

These days it is typically marked compressor oil or something like that so it isn't used in a modern engine. Power steering fluid that does not have leak stop, " 3 in 1 oil " or hydraulic oil would be fine. Be sure not use anything over SAE 20 as in cold climates the motor will run slow for a while.

I've used ATF and detergent motor oil before and it seems to bring a gummy residue to the surface of sintered bearings causing drag. Never use WD 40 , it isn't a lubricant for that type of application.