ISO68 is a viscosity index. It says nothing about the other properties of the oil. "Gear oil" could be a whole lot of different things, not all of them good for a mill. Assuming you are talking about a gear head milling machine, find some ISO68 hydraulic oil, preferably AW type (anti wear additives), or ISO68 spindle oil. Hydraulic oil is sold at Tractor Supply and tractor dealers. Spindle oil is available from machinery dealers and the usual suspects like ENCO, MSC, Travers, KBC, etc.