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You should not use a "hydraulic oil" in the N-series. These are for elevators, jacks, log splitters, etc. The lubricating properties are wrong for transmissions and rear ends. You can use a "tractor hydraulic fluid". This are intended for mixed use in hydraulic systems and transmissions. I prefer real FNH 134D to the "universal tractor fluid" typically avalable at big box retailers. The universal fluids claim "compatibility" with 134D, but have a noticeably lower viscosity, and thus more propensity to leak. You can also use an 80W - xx gear oil. The "extreme pressure" high-sulfur additives in the GL-5 spec fluids have been known to attack small brass or bronze parts (especially synchronizer rings) in transmissions under high-temperature conditions. With 5 gallons of fluid in a big cast iron tractor case, it is unlikely that the N-series tractor will ever experience "high-temperature" conditions. Furthermore, the only bronze part that I know of is the big fat PTO bushing -- the transmission has no synchronizers. I personally think the 134D fluid is a good compromise and use that. But I wouldn't lose too much sleep over the difference between GL-3 and GL-5 gear oils. "You pays your money and takes your choice..."
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