After agonizing about it for a bit I changed the transmission and differential fluids on my '99 M3 vert on Friday... (113,500ish miles)
I have 2 nugets to offer...
I did NOT need a stubby 14mm allen socket for the differential, a regular allen key (although 14mm was not so easy to find) worked fine. I was prepared to have to cut a chunk off it and use it in a box wrench but there was no need at all. and it was NOT on so tight that I needed a breaker bar or have to do chinups on it... I was actually nervous I may have been torquing the plugs back in too much.
The gear oil didn't look bad coming out but I think the redline 75W140 fluid is probably better stuff anyway. (I'll pretend I imagine it being smoother?)
The transmission fluid change was an obvious improvement... I had removed the CDV and replaced the clutch flex line with ss braid a few months earlier and was pleased with the results but have always found this car to be notchier than I like... I have another e36 (but '94 318iS) that shifts much smoother.
My transmission had the yellow sticker specifying MTF so I used redline MTL.
I didn't notice much difference for a day or two but it now shifts smooth as silk. I still think I'll replace transmission mounts before long just to further help avoid a money shift but it's less likely already now.
Happy that the job is done and as mentioned the transmission fluid change was definitely an improvement... I do feel like I could have saved the $45 on the diff but It's a maintenance item I can tick off the list for a few years now.
As an aside the 318iS diff fluid was black when I changed it... and the difference was immediate as I drove away... it's transmission will be due soon but I think it's almost clutch time because all of the action is at the top of the pedal so I may as well wait... I also doubt I want to tackle that job on my own :-(