Bought my Z from the original owner at just over 59,000 miles. Today I changed the differential fluid at just over 65,000 miles. The fluid was very dark, but not black, and definitely smelled burned. I suspect the fluid was the production fill. Additionally, the magnetic drain plug had a heavy coating. The coating came of with paper towels and had the viscosity of a thick paste.
Given the heavy coating on the magnet, I will be changing this fluid during my next oil chage. I'm sure there's still a decent amount of metal coating internal surfaces, and will be washed away by the new fluid.
If you have not changed the differential fluid in your Z, I recommend you consider doing so.
Check out the photos.
