Quaife recommend Silkolene 75W90 in their ATB Diff. I recall " TRANS X "? With NO friction modifiers. Roy Smart and Dave Parker have more knowlege of different lubricants ( all be it using Renault box's) with these ATB's
Audi recommends GL-4 lubericant for their transaxles, NOTHING else! Not a combination fluid like GL3, GL4, GL5 or anything else. A fellow explained it to me saying that GL-5 has twice as much molybdenum in it and would essentially come out of suspension and damage parts as particulates.
In any case, not sure about that but VW/Audi will not warranty new transxles unless GL-4 is used and only GL-4. So, I'm using Red Line GL-4 with no friction modifers as you suggest.
So, folks with Sudi transaxles might want to give this a try.
According to the Quaife product guide NO friction modifiers should be used with Quaife ATB's. They recomend Fuchs Silkolene "Silktran Syn 5" an API GL4/5 SAE 75W-90. Ron i don't know how this affects your Audi GL4 warranty dilema?