My bevel box has been howling at certain speeds. I called Morgan Motors of New England and they recommended Redline 80W250 GL-5 gear oil. Wow! I can still hear a soft whine of my drivetrain, but no bevel box howling. Thank you, thank you, thank you!
I am a bit surprised to hear that you had good results with this gear oil Sportster. A few of us in the UK clubbed together and bought a (US) Gallon of this to try about 5 years ago, it's not easy to get here. It even made claims in the advertising that it, "reduced noise of M3W bevel boxes" or something similar. All of our small test group found that it didn't seem to make any improvements and actually seemed worse that the Millars EP140 or Redline Heavy Shockproof 75W250 that we had been using before. It is very thick when cold and quiet, until the BB warms up.
I know it is fatal to get into "best oils" but several MX5 forums recommend Castrol Transmax 75W-90 fully synthetic GL-4 oil for the gearbox. Probably very similar to the Motorcraft oil, I will add that to the Alternative Parts List, on your recommendation. I can say that the Castrol certainly does improve the already good Mazda gear change and is worth using if you are changing the gearbox oil.
