I changed out my engine oil, 7-speed transmission fluid and the differential (non-Z51) fluid on Friday the 29th. Changing the tranny and diff fluid is easy. This work was done on my 4-post lift, which I checked for level before beginning.
For the tranny, I used Amsoil Torque Drive synthetic Dexron III. The Tremec has a published capacity of 3.7 qt, but after draining the oil and replacing the drain plug with a new magnetic drain plug* I was only able to pump in 3.25 qt before flow out from the fill hole occurred. So, I assume that about 0.45 qt is captured in the tranny and unable to drain.
For the diff, I was pleased to see that there is a magnetic factory drain plug. In fact, both the fill and drain plug on the diff are identical and both are magnetic. Since there have been issues with low diff fluid from the factory on some cars, I carefully captured the fluid for measurement. After complete draining (about an hour of slow drips) the total volume of diff fluid was 71 oz. The diff is spec'd to hold 91 oz (from Amsoil website), which is exactly how much I was able to install.
I've never had any problems with the diff over 4800 miles. The diff fluid was quite dirty, lots of fine metal, and the magnetic drain plug had a good coating of fine metal particles. This isn't unusual for the diff, given that a limited slip diff is a wet multiplate clutch system, and sheds metal from the friction plates into the oil. I replaced the factory magnetic drain plug with a new aftermarket magnetic plug* that has a much larger exposed magnet surface area and about twice the magnetic strength of the factory unit (tested by using the drain plugs to lift various weight box wrenches). I filled with the factory GM gear oil (GM P/N 88862624), which is synthetic and contains the necessary friction modifier.
I then took the car out for a quick drive to get a feel for the shifting performance. Cold shifts are smoother, but as the transmission warmed things seem about the same. Shifting quality for my car has always been good.
After getting the car to full temp, I drove roughly 30 miles, lots of shifting 1st through 5th, with light and hard acceleration. I then headed home and put the car back on the lift. I let the car cool for two hours and then checked the fluid levels. Both the tranny and diff were still warm, and after removing the fill plugs I saw a few drops of fluid emerge from both, indicating both were filled. As a double-check, and because the tranny only accepted 3.25 qt (versus the 3.7 qt spec), I tried to add another oz of fluid. The tranny wouldn't accept any more fluid, and was truly full.
I've put over 100 miles on the car since the fluid change. The car is running and shifting well. There is a subtle improvement in shift quality, mostly when cold, but the car has always shifted well. More importantly, I've got the magnetic drain plugs in place to scavenge metal particles. I placed it on the lift about an hour ago to double-check for leaks - all is good.
I'll replace the tranny fluid and diff fluid again at 25,000 miles (motor oil every 5,000 miles).
*Plugs were purchased from Custom Corvette Accessories
. The plugs are listed as C6 items, but fit the C7. The diff plug is longer than stock, but has no clearance issues - I carefully checked clearance before installation. Torque specs for the tranny fill and drain lugs are 20 ft-lbs. The diff fill and drain plugs have a torque spec of 20 ft-lbs.
Images from the Shop Manual: