Originally Posted By: MolaKule
Originally Posted By: redwolf4k
Hello,
This may be one of the most clouded topics in regards to gear oil. In 2017, is a GL5 MT1 rated gear oil acceptable in a thirty year old brass synchro manual transmission?
GL5 gear oil is cheap nearly everywhere, GL4 gear oil is mainly from specialty brands and expensive.
I wanted to ask the pros here: GL5 MT1 = yellow metal safe?
You're missing a key point here as explained in the linked posts.
Sure GL-5 differential lubes are relatively cheap but they don't have the proper Anti-wear additive mix, nor do they have the friction modifiers that make for smooth synchro operation.
And GL-5 differential lubes have a higher viscosity than specially formulated (dedicated) MTF's, which may make shifting more of a chore in cold temps.
If your transmission requires a 75W90, then Amsoil MTG or Redline MT-90 would work a heckuvalot better in your tranny.
This is a good point.
What I gather from everything here is both GL4 and GL5 is safe for use, but GL4 is the fluid I should use in regards to its functionality with a manual gear box. I will do as follows: Put cheap GL5 gear oil in my Diffs and transfer case, but I will order up some Redline MT90 for the tranny, as it is something I have to feel all day long while driving. I really hate spending upwards of $20 a quart for either of these. But I guess it is only once in a while.
Thank you for all the clarification!