There are some dual GL5/GL4 gear oils. I would expect they are fine for the syncros. Oil viscosity is only part of the smoother gear changes, friction modifiers are also used.
The people on the other forums like how butter smooth the shifts are with the Redline oil.As for myself right now I am thinking on a GL4, synthetic, 75w 90 Redline
I used 1L of Fuchs/Titan Sintofluid on my 01E 6sp as it meets VW/Audi specs VW 501 50 (G00500005 G005000 G00500020 G052911A1-A2).
Titan Fuchs Sinto Fluid, Advanced Automotion
It's a bit notchy on the 1-2 when it's cold, but nearly all 01E are notchy on 1-2 when cold. It's also GL5 rated. I am going to try 1L of a Chrysler GL3 that is known to be easy on syncros and see if it makes a difference in the 3 months I will test it. If no change, it's the trans and I will go back to Fuchs as it's highly recommended on the S4 forums . You can get the Redline or Royal Purple GL4 at many auto parts stores.
Here are the Audi trans specs (same as Passat): Audizine Forums
I'll say, the 6speeds are PITA to get into 1st when cold, and even then after it's hot, sure isn't as smooth as the 5speed box.I did use the Chrysler GL3 but it did not make a difference. I sold the car, not sure what's in it now. The Redline MT-85 should be fine, but I don't know if it will be notchy when cold. That was more of an issue in the 6sp trans like I had. I bought 6L of the Fuchs, but I don't know if I sold it with the car or still have it stashed in the garage.