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.
Ron