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Thanks again everyone for your comments. All is greatly appreciated. Mission accomplished. I replaced with 2 quarts of Royal Purple 85W-140 and 4oz of Bardahl gear seal. Interesting note...the copper crush washers I removed were smaller than the ones I received from M_x. I used the correct part# from M_x for what is supposed to be 7/8 OD x 5/8 ID copper crush washers. All in all pretty easy. The smelly dark gear oil I removed looked a little dubious. Thoughts are welcome.

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Old gear oil is the smelliest stuff ever. The smell with penetrate through nyrtril gloves.

I heard that the old used some kind of animal fat that turns rancid. Hence the smell.

Probably because no one makes 5/8 ID washers anymore and they sent you 3/4 ID.

Doubt it will ever make a difference as long as they're torqued to spec and somewhat centered.

Just a heads up. As I had no patience to wait. Our venerable vendor actually sent me 1.00 OD x 0.625 ID copper crush washers under part #433248. For $5.00 a washer. Today I received my copper crush washers from fittings.space/ dynapex. They were 7/8 OD x 0.625 ID and 0.75 cents a washer. These were an exact match to what I removed. Like Potato said, I hope this does not make a difference as long as torqued properly. The bolts do have blue monster PTFE seal tape on them. I drove in multiple parades this weekend. So far no leaks. Fingers crossed.

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It seems that leaks mostly come from the large case seal which is simply a huge square O-ring. I added it to the parts cross reference, FYI.

Side note, is the engine oil pan drain plug the same size and use the same size crush washer as the 2x plugs for the rear differential?

Yes, and this can be checked in the parts book

The same plug was used on nearly every engine, Ultramatic, and diff case 48-56

https://packardinfo.com/xoops/html/modules/partslist/index.php?mode=partnumber&SearchValue=426205&ButtonLookup=Go

Thank you. I wanted to make sure I wasn't seeing things when I looked in the parts book as I did see it multiple times.

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This past week when I put my rear axle back together, I used 75W90 GL5 oil that I already had on the shelf in my garage. At the time, I thought I would change it in a few days to 85W140 for "extra protection", but after reading more about viscosity and stuff in Richard Michell's oil handbook, I think I may actually stay with the 75W90. In 2005 the viscosity scales changed a little bit, and if I want to use a multi-viscosity oil, and stay around the same viscosity as packard specified, the 75W90 is actually a good choice. In the book it also mentions for classic cars that are in some competition or hard driving, a 75W110 would be a good choice when the original spec called for the old SAE90. The 110 being one step up on the viscosity chart I guess.
When I was younger, I ran 20W50 engine oil, but have changed my choice to lighter oil in my cars (10W40 in all my cars).
At about 140 degrees F, the 85W140 grade oil is about twice the viscosity as a 75w90 (100 vs 50 cSt) It seems like that might just be extra drag.
I guess all these oils will keep the rear axle alive and well, but it seems to me like the "lighter" 75W90 or 80W90 or 75W110 are close or slightly above the packard spec and will offer enough protection, and the 85W140 oil might just slow you down.

That is my opinion as well. If 90-weight (80W90) was fine when new, not much should change especially after a rebuild. Somewhat unlike engine oil, the main preventer of wear is not viscosity, but the EP additives which also give it the distinct smell.

The rear diff running hot could also have a detrimental effect on the shaft seals, especially the front seal which is pretty much directly exposed to splashing fluid.

Personal experience: I am tearing apart my rear diff now and it's very likely it had gear oil by the book, as most all of the mileage (93k) was put on in the first 38 years. While the side bearings are shot, the gears look great. Improper lubrication (e.g., using engine oil) will tear up the gear teeth first, as there is much less contact length than on a bearing. Also (and probably more important): bearings roll; gears slide.