I opened up my differential to find out the gear ratio and check the general condition of it. I haven't dismantled it yet. When I took the cover off, I was expecting more gear oil to come out than what did. I don't know offhand what the capacity should be. There is a varnish buildup that actually peels off. I am wondering if this a result of sitting idle for so many years, running it with a low oil level and overheating or something else? The pattern is looking good with minimal wear and there is no visable damage. Has anyone seen a film you can peel off in a differental before and if so was it from overheating? Should I just flush it out good?

It's hard to see in the pic where I peeled it off.

I've gotten a few diffs really hot and it doesn't leave a varnish or a peelable layer. The fluid is generally dark and watery and you might see some bluing on the ring gear.

Those spider gears are looking pretty nasty. Perhaps some moisture got in at some point and clouded up the lube, although at first glance it looks like baked on oil. I suspect that there would be other signs of overheaing as John suggested if overheating is the cause.

As far as low fluid, I once had a diff vent get clogged on my 4runner, and fluid started getting pushed out at the outer axle seal and corrupted my brake shoes. This is due to expansion as the gears heat up. That pressure needs somewhere to go. The brakes aren't as issue on the Z obviously, however this could account for a loss of fluid. In the case of the Z, it would come out at the output flange seal or the input shaft seal. The fluid level is determined by filling it until the fluid starts to run out the upper fill plug. It should be just beneath this when on a level surface.