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Next project. As I was underneath my car admiring my new gas tank, which unfortunately appears to have some scrapes and scratches I was buffing out, I noticed how sad my rear end differential was looking. It most probably has not been attended to in the last 24 years. Any processes, protocols and procedures would be helpful regrading cleaning this up with new gaskets, seals and modern synthetic DOT fluid. Keep in mind the car does have a GM 700R4 transmission. Thoughts are welcome.

Regards,

TC

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The service would be the same regardless of transmission type and the manual should cover it all other than the type of oil. I’ve used Royal Purple 01303 Max Gear 85W-140 High Performance Synthetic Automotive Gear Oil in manual transmissions but let’s let others weigh in on whether this weight is good for rear ends or not.

Many moons ago we used 90w pure mineral gear oil, in diffs, which is what it was designed for. You should be fine with 85/140, like was said above, or a RedLine "heavy" lube. Just a note of caution, the modern EP (extreme pressure) additives, over time, will degrade brass or bronze, so be wary of bushings
or maybe side gear spacers etc. in application. pb

EP rated lube is fine for a 54 diff.

Drain the old fluid, fill with fresh. Nothing else needed unless it is significantly leaking.

I added the part numbers for all the rear end seals in the cross reference except the front main seal. There is a square O-ring and an inner and outer axle seal in addition to the front main seal.

But like Kev said, if it's only leaking a bit and the seals aren't ancient you can just top it off. Could be worth checking for axle leaks though—just take off the rear brake drum.

I assume 80W90 EP gear oil is appropriate since the manual calls for 90 weight, or 80 weight in subzero weather.

I am going through my rear (53 Caribbean) axle now with bearings etc, and I think I will use 84W-140 GL5 when I put things back together.

I just replaced the seals on my 1951 Henney-Packard’s rear end and will fill it with whatever I have, which is the Royal Purple that I mentioned earlier. The car is not going anywhere other than around the yard so nearly anything I put into it is probably better than dry as it is now! Most if not all the oil came out of a bad wheel seal years ago before I bought the rear end but I’ve not yet pulled the drain to get the rest out..

85w-140 for the rear axle.

I agree with what Kev and HPH said, if there are no axle leaks and the leak from the differential is minor, I don't know that I would bother with tearing down the rear axle. Replace the fluid, and if you feel like making it look prettier, clean off the rust and paint it. It looks like the leak is only from the fill plug (and maybe the drain plug), so just put new crush washers on when you drain and fill the fluid.

If you really want to replace the seals, pack the bearings, and set the axle play, I did all that on mine and have it documented starting here. I had to do mine because the axle seals started leaking soon after I got the car moving under its own power.

Why use 85W140 instead of 90 weight?

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I don't think it matters much if it's 75w-90 or 85w-140. I've done both in my car and am currently using 85w-140. I think the bigger issue is to make sure the gear oil is GL4/GL5 or HP rated. Otherwise the gears can have same major issues.