Thank you for the link, did not realize this is currently being discussed.
Seems to be two camps here, which is why I came here in the first place. If only GL4 gear oil was cheap like GL5, and not becoming exclusive to specialty brands that want $20 a quart. I do not want to support such a price point for GEAR LUBE.
Doesn't the GL-5 specification in and of itself have yellow metal protectants? My ECHO owner's manual says either is acceptable and it has brass synchronizers. I've used Mobil 1 gear oil for a long time and according to the container it is GL-5 only.
1994 BMW 530i, 227K 1996 Honda Accord, 262K 1999 Toyota Sienna, 394K 2000 Toyota ECHO, 267K
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.