Seen anything from atf to motor oil to 90w gear oil run in them with success. Even the oil companys and oil blenders don't really see to have any consistency IMO. Used several "chaincase oils" from different companys, they have ranged from water thin (amsoil), to gear oil thick (yamaha), and everywhere in between. Some of the OEM oil (like skidoo) seem to use some sort of solids that are visable, moly or graphite, or something? in the oil. I suspect for some insurance that the chain has some lube on it if the inner seal leaks and you lose your fluid. Other fluids don't seem to have this. If you start looking a spec sheets for the different fluids you find no consensus in viscosity, many thick many thin, many seem to match another product in the companys line. So what I'm trying to say is even the specialty blenders don't seem to agree what "chaincase oil" is supposed to be. In my sled I usually use some outboard lower case gear oil, but this season I have some 15w40 delo in it (must have been the closest bottle on the shelf, LOL). IMO, for a 20 y-old scud, the lower seal may prefer something on thick end, like some gear lube like some 75w90. It's just a chain.