Red Line MT-90 75W-90 GL-4 Manual Transmission Fluid, 1 Quart

This is Red Line MT-90, a full synthetic 75W-90 rated GL-4, and the GL rating is the whole point. Most 75W-90 gear oil on a parts-store shelf is GL-5, which carries the high-sulfur extreme-pressure additives a hypoid differential needs but a manual gearbox does not. Those reactive sulfurs attack the yellow metal used in synchronizer rings and bushings, and over time they eat the synchros and turn a good-shifting box into one that grinds. MT-90 is formulated for the GL-4 spec, so it protects the gears without the additive package that chews up brass and bronze synchros. If your owner's manual or a rebuild spec calls for GL-4 75W-90 or a specialty synchromesh fluid, this is the fluid that meets it.

It fits a long list of manual transmissions and transaxles that specify GL-4: Jeep AX-5 and AX-15, older Toyota and Honda five-speeds, Volkswagen 02A/02J, BMW manual gearboxes, and many Muncie, Saginaw, and import boxes from the 1980s onward. Red Line sells the same fluid as part 50304 in the quart, 50305 in the gallon. One quart is a partial fill; most passenger-car manual boxes take roughly 2 to 3 quarts, so check your capacity before you order. It also covers GL-1 and GL-3 applications where a straight gear oil is specified.

The service is simple bench work: warm the transmission so the old fluid drains cleanly, pull the drain plug (or the lower fill/level plug on boxes that fill from the side), let it fully drain, reinstall the plug, then pump the new fluid in through the fill plug until it just runs out the level hole. A hand pump on the quart bottle makes filling a side-fill case far less messy. The synthetic base holds its viscosity hot and stays fluid cold, which is why the common report after a change is that first-to-second and cold shifts stop hanging up.

The most frequent reason techs and owners reach for MT-90 is to undo the damage-in-progress from someone having put GL-5 or an 80W-90 differential oil in a synchromesh box, and to quiet a transmission that has started grinding into 2nd or 3rd or feels notchy until it warms up. It will not rebuild worn synchros, but on a box that is merely fighting the wrong fluid it usually restores clean shifts. Ships from the USA.