I can't think of a good reason not to use a synthetic 75w90.... am I missing anything?
I don't put a lot of miles on my car and none of them are in anger so that should be good enough until i replace it again when i get an LSD....
I did more reading on Sierra and Locost forums, and there is no consensus it seems... except that straight 90 isn't available. On my dd last year I used Mobil 1 75w90; this time I debated between Redline Shockproof and Amsoil 75w90 Severe Duty. In the end I went with the latter.
One other thing - when I did my dd I got the advice to use RTV to seal it back up, but after having taken two apart I know that is not the right amswer. The proper stuff is going to be a flange sealant, like Loctite 518. I had some Permatex Permashield, which is essentially rebranded Hylomar Blue so I used that. I may redo my dd with Amsoil and Permashield... or like you I may just let it ride til I go in for that LSD.
Edit: Also, wow, changing the diff oil is a LOT easier without a pesky fuel tank in the way. With the car on jack stands I could basically sit under the spare tire well and work in a comfortable position.
I was working on the fuel pump when I noticed the diff cover was looking particularly nasty.
There was never gonna be a better time for me to deal with that. Popped the cover off and had one ugly, old diff. I suspect the vent tube may have become dislodged or clogged and let moisture get trapped in there. Not nearly as bad as the Falcon (which some of you may recall), but certainly not good.
I cleaned it out as best I could and let it drain overnight, then cleaned everything up and put it back together. It's cleaner now, and I used a little RTV to (hopefully) seal the vent tube in place... it's old and probably a bit shrunken. it seemed secure when I last looked at it.