Hello everyone, oils are one of my favourite topics. when i was dealing with the rep for our steel mill I asked about lathes and he said that is easy, hydraulic oil 68 cst viscosity. Explanation: hydraulic oil has a mild antiwar additive and lathes are typically lightly loaded. The more additive in an oil the quicker it will degrade from the additives = impurities. Also like someone had mentioned, some EP (extreme pressure) gear additives are not completely compatible. Generally, this information will conform with the original manufacturers recommendations . Motor oil is an accepted alternative to hydraulic oil where availability is an issue. A higher viscosity will give a thicker oil film and more protection, but the cost is heat generation and resistance, a lathe gearbox is typically over engineered for accuracy reasons, hence the light loading.
Happy machining my friends.