For racing should I use a higher viscosity like 75-140?

No one has mentioned Valvoline GL5, is it good to use in the diff and transmission of a 1984 GSL-SE?

For the diff a good choice would be Amsoil Sevear Wear 75W-110.
For the trans Redline 75W-90NS is GL5 which is better than the GL4 MT90.

You have to consider that many of us are many a lot more power than the stock system.
Just as we need better cooling we need better lubrication.

Have you looked into Amsoil! I'm currently using it in my 85 gsl-se and 91 turboII. Both my trannys also (turboII both) Awesome oil no complaints. check your year S4 or S5. Amsoil GL-4(thinner) GL-5 (thicker).