And those are both engine crankcase service classification viscosity grades, not gear oil. An 50 on the SAE crankcase scale is in the same range viscosity-wise as a 90 on the SAE gear oil scale.d8h like mine uses 30weight engine oil in finals or warm weather 50weight according to lube chart on top of dash
That's not entirely true, cat tractor scrapers still use 90 wt. in diffs and final drives with duo cone seals.Another thing to remember is the the additives in EP gear oils have been known to degrade the silicone rubber in Duo-Cone Toric Rings and cause premature leaks and/or seal failures
Cat strongly advise against the use of EP oils in any compartment that has a Duo-Cone seal in it. The attached Service Mag for excavators is just one example.
50 wt lube oil has been the recommended oil for D8H&Ks finals since at least the late 60,s so if you are using 90 wt you are causing the final drive seal failures.That service letter refers to excavators with planetary drives..as do haul trucks which have silicone toric rings. Gear oils have a phosphorus additive which is a smelly lubricant enhancer which spur gears need because of extreme pressure as single gear teeth slide into each other. Planetary gears can get by without it because the force is spread over more teeth.