The Haynes MR2 repair manual suggests changing the manual transaxle fluid every 24K miles or 24 months, whichever is sooner. The fluid should also be changed if shifting feels irregular or unnatural. The Toyota MR2 owner’s manual recommends 2.7 quarts of SAE 75W-90 or 80W-90 gear oil (API GL-4 or GL-5) for the 4A-GE engine (normally aspirated). Red Line MT-90 is such an oil that is highly recommended for the MKI MR2 on the MR2 forums.
Tools: 24mm socket (6 point recommended), 1/2in breaker bar, 1/2in ratchet, torque wrench, drain pan, and a small fluid pump (see Lubrimatic Fluid Quart Pump 4cc on Amazon.com):
First locate the fill and drain ports on the transmission (fill highlighted in red, drain highlighted in yellow):
Next, use the 24mm socket paired with the breaker bar to loosen the fill port plug (make sure to open the fill port first). Once the fill plug is removed, use the socket to loosen the drain plug. Both plugs are accessible without needing to jack the vehicle. Before removing the drain plug, place a drain pain underneath the drain port to capture the old transmission fluid. Make sure not to lose the crush washers that are associated with both plugs (or better yet, replace with fresh crush washers: Toyota P/N 90430-A0003 – same for fill and drain). Once the old transmission fluid has drained out, install the drain plug with crush washer and torque to 37Nm (27ft-lbf).
Next, use the fluid pump to fill the transmission with fresh fluid. Fill the transmission until fluid starts running out of the fill port. When the transmission is full re-install the fill plug with crush washer and torque to 37Nm (27ft-lbf). Start the vehicle, take it for a short drive, and check for leaks periodically.
(Updated 2019-10-19: added fill and drain plug crush washer P/N and added plug torque spec.)
Mr. Tu's MR2 