Is gear oil the same as differential fluid for a Mazda RX-7 FD?

While both the manual transmission and the differential in your FD RX-7 use a type of gear oil, they require different fluid formulations and additive packages.

Here is the breakdown for your vehicle:

Transmission Fluid The manual transmission requires a standard 75W-90 gear oil. Redline MT-90 is widely accepted as the standard, highly recommended choice for the FD's manual transmission ⁴.

Differential Fluid The Factory Service Manual specifies a GL-5 75W-90 gear oil for the differential, or an 80W-90 weight if you are in a warm climate ⁴.

When selecting a differential fluid, there are a few critical rules for the FD:

• No Friction Modifiers: All FD RX-7s are equipped with a Torsen Limited Slip Differential (LSD). You must avoid gear oils that contain LSD friction modifiers, as they do not play well with the Torsen internals ⁴.
• No Shockproof Oils: You should also avoid "shockproof" style gear oils in the rear end, as they are known to gum up the Torsen system ⁴.

For the differential, a standard GL-5 75W-90 gear oil without these additives—such as Redline 57904 (75W90 GL-5) or an equivalent Idemitsu gear oil—is the correct choice ⁴.