> I might have missed this but what is desirable about GL-4
> versus GL-5, etc.
Hmm, guess my previous rant didn't go through. Too big maybe.
In a nutshell, there is a sulfur additive in both GL4 and GL5 that breaks
down and form sulfuric acid, which can attack the "yellow metal" used in
synchro rings, thrust washers, etc. Conventional GL5 typically has more of
this additive (like twice as much), but GL4 uses it as well.
However, the GL4 and GL5 standards say nothing about the additive itself, so
not all GL4 and GL5 oils are created equal. In particular, full synthetic
oils typically need less of the additive to meet the performance spec; so
many GL5 synthetics are considered safe for "yellow metal".
There are even some oils that meet both GL4 and GL5 specs; one is Valvoline
Durablend and another is Royal Purple. It's my belief that many more
_could_ meet GL4, but their makers just choose not to perform the tests or
put the logo on the bottle. For example, Sta-Lube used to sell a
conventional gear oil that was advertised as meeting both specs, but they
don't any more.
> Does it just say that both are adequate for
> this application but GL-5 is not no matter what brand?
I believe there is a lot more to gear oil than whether it meets GL4 or GL5.
In fact, I would say that the MT-1 designation is more important than either
GL-4 or GL-5, for use in a manual transmission.
Randall
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