You should always be safe to run ISO 68 oil in your gear boxes. I would be sure to buy one specified for gear oiling, since they come in flavors. It's a multipurpose oil. Hyspin 68, Tellus 68, Rando HD-68, DTE-26, HP-68 - they are all interchangeable oils with 68 centistokes viscosity at startup temp. I have yet to see a lathe that doesn't spec this stuff. Maybe some of the far-out hydraulic lathes out there that do more than splash lube with oil might spec a lighter oil for those exotic drive systems. But your run of the mill geared head lathe is going to spec some form of ISO 68.