What is the recommended transmission and differential oil for a Mazda RX-7 FD?

For your FD RX-7, here are the correct fluid specifications and recommendations based on the factory specifications and proven real-world applications:

Manual Transmission:

• Specification: Per the Factory Service Manual, use 75W-90, or 80W-90 if you are in a warm climate. The fluid should be GL-4 or GL-5 rated ².
• Top Choice: Redline MT-90 is widely accepted as the standard replacement for the FD transmission ². Do not use Redline MTL, as it is a 75W-80 and does not meet the factory specification ².

Differential:

• Specification: Use a 75W-90 GL-5 gear oil ².
• Critical Torsen Warning: All FD RX-7s are equipped with a Torsen limited-slip differential. You must strictly avoid gear oils that contain LSD friction modifiers or "shockproof" additives. These additives do not play well with the clutches in the Torsen differential and will gum up the internal components ².
• Top Choices: Redline offers a 75W-90 GL-5 without the friction modifiers (part #57904), and Idemitsu also makes a highly recommended gear oil suitable for the FD differential ².

Difficulty Rating: Easy (Standard drain-and-fill procedure for both components).