posted on Jul, 6 2019 @ 11:21 AM
Gear oil is GL-5 made for diffs and contains friction modifier additives.
GL-4 spec MTF manual trans fluid is for transmissions and contains no or less friction modifiers so the synchronizers can grab each other.
Many older trans took 10w-30 motor oil, but you can't use "API Starburst" energy-efficient motor oil for the same reasons, so....
Pennzoil made a specialized replacement called "Synchromesh", GM brands their own. At 10cSt it's much thinner than 75w-90 gear oil at 15cSt, replacing
any and all MTFs. Some oems use incredibly thin MTF, thinner than ATF around 6cSt, so going "up" to Synchromesh OR "down" from a 90 weight to
Synchromesh, is no problem, it's well advised and standard practice for the last 10 years....new to old shop mechanics. Google it. Remember, old
school gear oil sheers to a great extent, so using a modern product that won't sheer, but starts out thinner is the same exact thing (when talking
with old mechanics).
Bottom line is PZ Synchromesh is awesome. State-of-the-art formulation replacing everything except GL-5 75w-90 GEAR OIL. It's all about the friction
modifiers and your synchros, the viscosity can be lowered or raised depending on engine tune, and mostly climate. GREAT in cold. In extreme cold,
people used ATF in their MT so they could shift. Again here, you don't use the friction modified ATFs, like ATF+4 or Universal....use Merc V or pref
Anyway, PZ Synchromesh, it's awesome and dirt cheap. Redline has some MTFs too, from a thin 10cSt 75w-80, to a midweight 12cSt "85" so you don't have
to mix your own, to MT 90. Some decent white papers on their site too.
edit on 6-7-2019 by FlyingFox because: "Round Her Up!"