Please don't use anything but gear oil, like 80W-90 in a gearbox or axle that specifies it. Gear oils have viscosities similar to engine, hydraulic, and power train oils but those oils do not have the EP (Extreme Pressure) additives found in gear oil. Some gears have sliding contact or extremely high contact pressures and gear lubes have additives containing chlorine, sulfur, or phosphorous phosphates that make it work. Conversely, some of these EP additives will erode clutch packs - power train or hydraulic oils are used in those applications. Hydraulic, power train and engine oils have anti-wear additives that are good for low pressures but they can't compare with EP additives. Along with 80W-90, there are alternatives that probable weren't available when your machine was built like 75W-90 synthetic, 85W-140 and 80W-140 that will suit your need. My predecessor made the mistake of specifying a gear oil in an axle with bronze faced friction disks got the brake and I had to write a multi-million dollar program to replace all the brakes and fill with a synthetic. Likewise he also specified a hydraulic oil where a gear oil was needed and as a resulted in wiping out several hundred limited slip differentials that needed the EP additive.
