not sure what's specified but i always used low viscosity oil. my meister is still alive and well. the only real issue i had with it was belt wear. be sure to clean the handpiece with naphtha or such after each use.
10w is the recommended oil unless it's low temperatures, then go to 20w. It's in the manual, you can get it from the GRS site, in the support section. There is also a note about using Dexron ATF if oil sludge is a problem. I use ATF in mine, without any issues. You might also need to either replace or clean the filters in the intake side. Refer to the manual for the Gravermeister noted above for more information. Great machine.