Royal Purple Synchromax Manual Transmission Fluid, 6 Quarts

Royal Purple Synchromax, part number 01512, is a synthetic fluid for manual transmissions and transfer cases that specify a light manual transmission fluid or an ATF-type fill rather than a heavy gear oil. The complaint it usually gets bought to solve is a box that grinds or baulks going into second when it is cold and clears up once everything warms through. That is synchro drag, and it comes from a fluid that is too thick cold or that has lost the friction characteristic the synchro rings need to bite.

Synchromax is built to stay thin enough at ambient temperature that the synchronizers can do their job on the first shift of the day, while still carrying an anti-wear package that protects the gear teeth and bearings under load. Being synthetic, it does not shear down into a thinner grade over a season the way some conventional fluids do, which matters in a box that sees track days or towing.

Before you fill, confirm what your gearbox actually calls for. Plenty of manual boxes and transfer cases specify a light MTF or ATF, but others want a GL-4 or GL-5 gear oil, and the wrong choice will either cost you shift quality or attack the brass in the synchros. Six quarts covers most manual transmission fills with fluid left over for a transfer case. Fill through the fill plug until fluid weeps back out at the plug with the vehicle level, and pull the fill plug loose before you drain so you are not left with an empty box you cannot refill.