With over 200k on the odometer, I planned on changing the Miata's transmission and differential fluid. While both the rear end and the engine were supposed to be new, and one of the previous owners was supposed to be a mechanic, changing these fluids is cheap insurance against abnormal wear or complete failure. You don't want old, dirty, or contaminated fluid lubricating the the drivetrain -- just as you wouldn't want it in your engine.

What do you need? Not much. Here is what I used:

• 24mm wrench or socket for the drain plug (both trans & diff)
• 14mm wrench for the transmission fill plug
• 22mm? or 7/8 socket or wrench for the differential fill plug
• Drain pan
• Fluid pump to fill both trans & diff
• 2.1 quarts transmission fluid -- I used RedLine MT-90
• 1 quart differential fluid (actually need a little less than a quart) -- I used Redline 75W90

Note that there seems to be lots of discussion on which transmission or differential fluid is best for the Miata. This largely seems to be a discussion of which fluid feels the best, as both Redline MTL and MT-90 do an adequate job protecting against friction, but each feels slightly different dur to the fluid characteristics.

I have a limited slip rear end, which influenced my decision as to which fluid to use. You'll see that I used Redline 75W90, not 75W90NS, and has limited slip fiction modifier. My understanding is that the friction modifier makes the fluid friction greater, increasing clutch lockup, but also potentially increasing clutch chatter. If you want the clutch-type limited slip rear end to have greater lockup, you can use the 75W90NS oil. Note that this applies to clutch-type rear ends only, not the torsen.

I did ask Redline technical support for their advice, and here is what they said:

• In the Miata transmission either the MTL, MT-90 or 75W90NS will work. I would recommend the MT-90 as that is the viscsoity called for, the MTL is lower viscsoity than called for, though does appear to work OK. I would expect the shiftability to be slightly better with the MT-90 than the 75W90NS. I wouldn't expect an improvement in protection with the 75W90, plus it is likely too slippery for the synchros.
• In the rear differential I would recommend the 75W90, suitable for limited slip or open differentials. If you wanted maximum lock up from a clutch type limited slip, the 75W90NS would provide that, though chatter would also likely be present when making a slow/tight turn.

You'll want to do your own research on oil viscosities, synthetics vs. crushed dinosaurs, brands, etc. Probably best to go to miata.net or usenet and do a little research before deciding on what you use.

To start, jack the car up and put it on jackstands. Put the drain pan under the drain plug, and use the 24mm wrench to open it up. If you want higher flow, you can use the 14mm wrench to open up the fill plug on the driver's side of the trans. With the drain plug out, inspect the magnetic end of it. Some fine material is normal, but if you see large chunks of steel, that may be a sign of impending failure. Clean it off before replacing. Replace the drain plug, torquing it to the correct spects, and begin filling. I used a cheap-o pump designed for this purpose. The pump has an extension to reach into quart containers, but I removed mine because it doesn't fit under the car with it on. Instead, I fit the whole body of the pump into the quart of oil by cutting the top off. Anway, fill the transmission with 2.1 quarts of oil. You'll know it's full because the fluid will start coming out the filler. Then replace the filler plug (coating it with permatex if you wish), and you're done with the trans. If you wish, you can replace the tower lubricant at this time, but I didn't.

Replacing the diff fluid is almost exactly similar. Pull the plugs, drain the fluid, and replace it with the new. The rear end is perhaps easier because it is more accessible, as well as taking less than a quart.

It appears that the fluid change has made gear shifts a little smoother. I don't feel any difference from the rear end, but I never had any chatter. I would like to think that the clutches hold a little better, but the Miata isn't real prone to spinning the tires to allow me to test. Either way, I should benefit from better component life & possibly slightly less friction losses. For $30, it's cheap insurance.

Finally, I should say something about oil recycling. With the impact oil can have on the environment, most notably our drinking water & the runoff water, you should not dump it down the drain or even into the ground. It will eventually make it into the water. Recycling centers are everywhere (California has a list of centers -- check your state for a list). Just bring the jugs into a center and recycle it. Or even better, if you have a diesel truck, you may want to filter it & run on it!