Tranny/Diff Fluids-- GL4 vs. GL5 spec.

[Jpinkowish@aol.com]

Hello Folks:

I'm having no luck finding a name brand trans gear oil that is GL4 spec.  
Lots of GL5 products, though.  The folks at Blau tell me they use the Castrol 
synthetic(which is GL5) for the trans.  Even though the Bentley says use a 
GL4 oil, is a GL5 spec product okay for the manual trans?

On a more general note for heavy weight gear oils, how important is it to 
strictly adhere to the specification.  Typically, with motor oils, a spec 
designation containing a higher number supersedes previous specs and 
indicates higher performance.

Comments/advice for both questions?

Jan Pinkowish
'85 4kq