Here is why one must always go through a vehicle unless you have receipts for previous work done. This is the 71's 2.56 rear when I opened it yesterday to a oil change! Yep I'm willing to bet that is the original gear oil as the cover still had a paper gasket and the fill plug was a PIA to remove. I used 2 cans of break clean and still didn't get it all of the gears. I could scoop it like Hershey syrup. Now hope the rear doesn't fail because the new stuff cleans out the old.

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Mmmmmm yummy... And, the aroma...!

 

One bad thing with oil is if you leave it sit around it gets waxy, especially in cold weather. That contributes to sludge. This is one of the major reasons you shouldn't leave cases of oil sitting in your basement or garage. Buy what you need, use it and repeat next time. This happens less with synthetics but still not a good idea.
That being said I don't think you have to worry about much by cleaning this out. It's only being splashed around and not pumped through anything in the differential. Making this fluid again can only help. My concern would be the sludge left in the axle tubes. That thing needs a good bath with kerosene.

 

Now hope the rear doesn't fail because the new stuff cleans out the old.

back in the day I heard many people say ''dont change the tranny fluid leave it alone'', because when you changed it a lot of those tranny's failed. something to do with a no combustion situation, i'm not sure, It always sounded strange to me and the only thing i could come up with is that slippery metallic paste is holding everything together and everyone is happy