Are Staral still around as a company? If you send them the serial number, they might give you some info. Do you know what spindle nose (and perhaps spindle taper) it is? There are some other Staral manuals available at usinages if you dig around.
The PTO shaft on old tractors often spins slowly even though it is out of gear. It is, as you say, due to the oil drag. Think how a torque converter (fluid flywheel) in an automatic gearbox works. Run it for a good while and make sure the oil is warmed up and see if it still happens. In winter weather, in a cold workshop, even the correct spec. oil will be on the thick side. Worst case, change the oil for the correct one. Does it have a spindle brake integrated with the clutch? Engage that if necessary or check it is working properly.
It could be the clutch plates binding due to overheating and warping. It would probably be worth checking out. If it's a wet clutch then its designed specifically to NOT drag and turn the spindle when not engaged, since its a safety issue!
It shouldn't be a big problem though, and might get away with cleaning and adjusting. It could even just be the wrong spec of oil, if its a wet clutch.
Sounds like you got a good deal
No brake on the lathe as far as i know, there is no marking for the oil level on the lathe, but when filling the oil till half way the sight glass (assuming this is correct?) the clutch is not submerged, it does however get absolutely drenched in oil when the lathe is running so am quite convinced the oil plays a major role in the sticking of the clutch. I'd rather not take apart the clutch as finding spares is going to be near impossible and i have no documentation as how to take it apart and set it up again. Will probably make it worse than it is now to be honest, will try with an oil change with correct spec oil first.
The correct oil according to the manual is index 95, viscocity S.A.E. 20, anyone know what that translates to into modern day oils?
Also found the manual already on the Usinages website, it's even in Dutch .