Leaking final drive gasket--can I substitute grease for standard gear oil until I can replace the gasket?
As long as it is a wet grease so that it flows, it should be fine. Definitely don't want a stiff dry type grease.
I don't recall any final drive gasket on D6C FD housings. The housing simply uses rubberised sealant/RTV to seal the joint. Are you getting a leak between FD housing and the main steering clutch/bevel gear case, or is the leak coming from around the sprocket? If the leak is coming from around the sprocket, then these are metal face seals, and they would be damaged in a big way to leak. If the leak is coming from between housing and case, then I'd suspect loose housing bolts. Either way, using a semi-liquid grease is only going to be a very temporary fix, and any grease that is liquid enough to run is going to seep out of the area where the leak is now, when it gets hot. By far the greatest concern is that gear oil contains Extreme Pressure additives to counter any potential spalling of gear teeth, especially on the bull gear, under heavy load. I don't know of any semi-liquid grease that contains EP additives. There may be some, but I can't recall any, right at present. Using a semi-liquid grease with no EP additives is going to risk serious damage to bull gear teeth.
Using a semi-liquid grease with no EP additives is going to risk serious damage to rare & expensive bull gear teeth.
Isn't it 50wt TO4 that goes in finals?
Yes, I believe that is correct. Oils are a constantly-changing product, as new additives are discovered, new oil types are produced, and oil recommendations are changed. I extracted this useful information (the 2 paragraphs below) from a Castrol site ... Cat FD-1 – Intended for final drives and axles in Caterpillar equipment types that do not contain friction materials (i.e. wet brakes). A Cat FD-1 SAE 50 or SAE 60 mineral oil-based product permits a 4,000-hour drain interval and an 80W-140 multigrade synthetic or part-synthetic product permits up to a 6,000-hour drain interval – if the operator is utilizing a used oil analysis program, such as Castrol Labcheck. Common applications include final drives in crawler dozers, as well as differentials, final drives and front wheel bearings in Cat off-road haul trucks. Cat TO-4 – The SAE 50 and SAE 60 grades can be used in final drives and axles with a 2,000-hour drain interval. Common applications are differentials, final drives and planetaries in Caterpillar wheel loaders, wheel dozers, excavators, and landfill compactors. TO-4 fluids are also used in transmissions and hydraulics in some machines in lower-viscosity grades. There are two current API specifications for gear oils - GL-4 and GL-5. Both have EP additives, but the GL-5 oils have a considerably higher level of EP additives to counter the extreme pressures found at the contact points of heavily loaded gears, such as ring gears and pinions in heavy duty axles, and the gears in final drives. I had a rethink about EP additives in grease, and checked on available greases, and I did then recall that there are greases with EP additives. However, after a little more research, it appears the level of EP additives in greases barely rate at GL-4 level. According to the information I gleaned, GL-4 gear oils contain 4% EP additives, and GL-5 gear oils contain 6.5% EP additives. All the EP greases I found appear to contain levels of EP additives that are between 1.5% and 4%, thus indicating that the vast majority of EP greases either don't meet, or only just meet, the GL-4 level of EP additives.
Maybe something like JD corn head grease or an OO grade grease that thickens when not in motion so it doesn't leak but turns liquid with movement. The JD grease is very popular and recommended for gear boxes that tend to leak. Might be a little expensive depending on how much you need. I used OO grade grease in a leaky track roller.