My miata started out as a base '91 model. There were two main weaknesses of my diff:
A) No limited slip feature: The diff in a base model miata is a 4.30:1 open diff, that is, if one wheel starts to spin, the other tire just gets less torque and the spinning tire keeps spinning... a real performance burden in tight turns.
B) Weak 6" ring gear and pinion set: All 90-93 diffs have a 6" ring gear and a relatively small pinion gear which have proven to not take too much load without failing. Aggressive driving styles and/or dramatic power increases to the gears can cause a tooth to break.
Well, after my 2 year life with the Aerodyne at various boost settings and a few months with the FM3 turbo at 15psi,... and one night of some overly spirited driving... my diff gave up the ghost. The failure was first noticeable by a slight popping noise with rotation of the rear wheels (regardless of engine running, regardless of which gear I'm in). I knew the diff was dead, and upon removal and inspection, I was right. I chuncked a few teeth off the pinion gear and really hosed the mating ring gear.
The funny thing about my failure was that the night of driving the car hard showed no signed of trouble all the way home. The next morning on my way to work, I began to hear a little noise. I had just put in new tranny fluid and figured I was a bit low and was hearing some gear noise. So... I kept out of boost all the way to work. During lunch, I took another miata friend out to hear the "noise" but this time I gave it some boost. BAM! Yep, there I was at work with a crippled miata. It was barely drivable so I immediately left work and limped it home through some low speed neighborhood roads.
Getting the diff out really wasn't too hard with the exception of the hub nuts being hard to break and start turning and to slide the splined half shafts out of the hubs. I had borrowed some 3/4" and even 1" drive socket wrenches from a friend which made the stubborn fastener break free. Still, a big torque arm was needed to get the nuts to budge. I hooked some extensions together to make a long torque arm. It worked!
Once the Torsen went in, the car felt more planted in turns and when I would get tire spin, it was more rare but BOTH tires whenever it happened.
Regarding fluids, I use Redline MT90. The drain and fill plugs are 23mm.