New-to-me '99 Miata - fluids?

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Jan 7, 2009
Rochester, MI, US, World
I just bought a new-to-me '99 Miata, very nice condition smile no pictures yet, since a true Bitoger worries about fluids first. The vehicle has been well maintained, but the manual gear fluid and differential fluid need to be changed. For transmission oil, the OM states to use a GL-4/GL-5 75w-90, and for the differential, the manual only states GL-5 (no grade given). Any suggestions? I've had good success running Redline fluids before in a number of vehicles. The car will only be driven in warm weather and garaged in the winter.
If you are happy with Redline, go with that. My choice would be Amsoil.
GL-4 only in the tranny. Redline is fine but the Ford fully synthetic transmission fluid is better. GL-5 only in the differential. Any good 75W-90 is fine but a good synthetic is better. Go over to and check out their lubricant subforum; they're the experts in all things Miata.
For the diff, any 75w90 will do just fine. I have used valvoline, Mobil 1, etc. All good. In the tranny, I would stay away from any 75w90 that has limited slip added. I have used redline, amsoil, even Supertech. All good. In ur case, Redline is perfect if u like that brand
Many Miata NB owners like this Ford fluid in their trans. Genuine Ford Fluid (XT-M5-QS) Full Synthetic Manual Transmission Fluid. I used it in my NB2 6-speed and it seemed to smooth out cold weather shifts and a notchy 1-2 shift.
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