The Student[1] says it should run on Shell Tellus 27. I doubt that this will be readily available. I noted today that the level is a bit down on the sight glass, although it still has oil in it.
What else is suitable and will it mix with whatever's in there (which might even be shell tellus 27 or equivalent), or do I have to try to find out how to get the oil out of it and refill it?
I've seen disrecommendations for running it on too-thick oil in an attempt to reduce the leaks, and unless the oil is very expensive, I'm not bothered at the small amount this one leaks, keeps the rust at bay.
[1] not student, which of course run on junk food and beer.