Final update:
- I tried Mobil 1 75W90 GL5 for <30 mi.-- it was completely unworkable.
- I tried Redline MT90 75W90 (GL4, but not recommended for hypoid gears) for <50 mi.-- it was a little better, but still not very good.
- I tried Brad Penn GL4 80W90 GL4 for about 250 mi.-- it was sufficient, but still tight on downshifts
- I put the rest of the Swepco 201 80W90 GL5 from my stash in tonight-- it was transformative.
I'll never put anything else in a VW transaxle again.