Any light engine oil will work.
3-In-One oil can or
saved last drops of engine oil from oil changes. Your generator might have one or two lubrication "cups" (spring-loaded caps on top of a little filler tube) where a few drops of oil are dripped in once or twice a year. No muss, no fuss, if you only put in a few drops. After dripping in the oil, wipe around the spring-capped filler tube. Yes, you might need to use contortionist moves to get to the rear filler cup/cap.
Some generators have no filler in the rear (no service lubrication). The rear cup/cap might have become eliminated in later generators because it was/is so hard to get to (except from underneath using a blind-fill move)?
A rebuilt generator might also have sealed bearings in the front (no service lubrication, maybe - the re-builder will advise).
