gear oil - GL-4 or GL-5?

[wc701lists at bellsouth.net]

GL5 has some kinda additive (sulphur compurnds, IIRC) that are corrosive 
to the copper / bronze components in the transaxle.  GL4 does not have 
them.  GL4 is hard to find in cheap gear oils, but I think Castrol used 
to market one.  I always used Redline - think it was MTL-90? Noticeably 
better shifter feel, esp in cold Ohio mornings where I used to be, 
though that's not a CA issue.
   -Wayne

On 3/13/2012 1:23 AM, David Pertuz wrote:
> My former G20, now owned in California by a friend, is getting a new
> clutch tomorrow. I was telling him some things to be aware of (to
> mention to the mechanic, really, since he is not the faintest car
> person) and I cannot remember whether GL-5 is OK for the trans in those
> cars. I remember that the service manual calls for GL-4 - I'm certain
> this is correct. I also remember that GL-5 may be bad for the synchros
> in the long run, but I'm not certain of that. I also remember that when
> I replaced the engine in the car years ago I absolutely could not find
> GL-4 anywhere, and ended up using Castroil Hypoid C, which was GL-5 and
> obviously worked OK since it was in there for ~80k miles and it always
> shifted great. Is the correct weight of GL-5 oil OK for the trans? I
> might have put Redline MT90 in there a few years ago (I can't remember!)
> but this is not a situation where anyone is going to be putting Redline
> in there.
>