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

This is the one-gallon jug of Red Line MT-90 (part 50305), a full synthetic 75W-90 gear oil rated API GL-4. The GL-4 rating is what matters in a manual gearbox. Most 75W-90 on a parts-store shelf is GL-5, built with the heavy sulfur-phosphorus extreme-pressure package a hypoid rear axle needs. Those active sulfurs attack the brass and bronze in synchronizer rings and bushings, and a box run on them long enough starts to grind. MT-90 carries GL-4 level protection without that additive load, so it is the right call wherever a manual transmission or transaxle specifies GL-4, GL-3, GL-1, or a synchromesh-type fluid.

The gallon makes sense when one quart will not finish the job. A lot of manual boxes hold more than a quart or two: many 5- and 6-speed rear-drive units run in the 3 to 4.5 quart range, and a shop or anyone doing a flush-and-refill after a rebuild burns through quarts fast. Common GL-4 boxes people put MT-90 in include Jeep AX-5 and AX-15, NV3500, older Toyota and Honda five-speeds, VW 02A and 02J, BMW manual gearboxes, and many Muncie and Saginaw four-speeds. Always confirm the fluid spec and capacity for your exact transmission first; boxes that call for ATF, a Dexron-based fluid, or a dedicated OEM synchro fluid should stay on that spec.

Service on the bench or under the car: warm the transmission so the old oil drains fully, remove the fill plug first (so you know you can refill it), then pull the drain plug and let it empty. Clean the magnet on the drain plug and check the fuzz on it, reinstall with a new washer, and pump MT-90 in through the fill hole until it just starts to run back out the level port. A hand pump or squeeze-bottle transfer from the gallon jug keeps a side-fill case from turning into a mess.

The typical reason to reach for it is a box that shifts notchy until warm, hangs up going into 2nd or 3rd, or was filled with GL-5 or 80W-90 differential oil by mistake. Fresh MT-90 will not fix worn synchro rings or a bad blocker, but on a transmission that is simply fighting the wrong oil it usually brings cold shifts back. Not for hypoid differentials or any application that requires GL-5.