[Fot] GL-4 question

[Randall]

> I might have missed this but what is desirable about GL-4 
> versus GL-5, etc.
> for use in a Triumph (TR6) tranny? 

As I understand it, the concern is an "extreme pressure" (EP) additive that
contains sulfur.  As the additive does it's job of protecting gear surfaces
after the normal oil film has broken down, it also deteriorates and
eventually forms sulfuric acid.  (This is also what gives old gear oil that
distinctive smell.)  This acid in turn can slowly corrode surfaces made of
copper alloys (collectively "yellow metal") that also rub, like synchro
rings and bushings.  If you've ever taken out a synchro ring that was black
where it didn't rub, that is a sign it was attacked by the EP additive.

Unfortunately the "GL" standards do not address this additive directly and
thus different formulations can have different amounts of it.  Even
conventional GL4 oils have some of it, just not as much as most conventional
GL5 oils.  It's even possible for a GL5 oil to meet the GL4 standard, for
example Valvoline Durablend gear oil.
http://www.valvoline.com/products/Durablend%20Gear%20Oil.pdf
(I believe Sta-Lube also used to market a conventional oil with both
markings on it, but they stopped due to consumer confusion.)

Also, since synthetic oils have a stronger base stock (meaning the normal
oil film will hold up under more pressure), they typically do not use as
much of the sulfur-based additive.  Thus a synthetic GL5 should be safe for
yellow metal.  At least I've been running GL5 synthetic in my TR3A
differential for many years now, and while the thrusts were pretty badly
worn when I got it in 1982, they don't seem to have gotten any worse in all
those years & miles.
(This isn't what I use, just an example of a GL5 full synthetic gear oil
that specifically mentions being safe for yellow metal)
http://www.royalpurple.com.au/files/CIY7IBY5U1/mgearaps.pdf

Randall