CircuitSmith…
I find it quite interesting that they mention both the GL4 and GL5 interchangeably. My understanding is that they are quite different, as you have mentioned re. the sulphur content in the GL5 and how this affects the brass synchros. Do you happen to have the Factory Service Manual to crossreff what they say in there?
My 2000 Nissan Altima says it needs a GL4 oil and I have been using the RedLine Synthetic MT-90 75W90 GL-4 in it since 2002 with very good results. You might want to check it out here:
I have also been using RedLine GL4 Tranny oil in my 1986 Golf GTI since 1990 with no issues at all. My shifts into 2nd gear are still buttery even after 225k miles 
Personally, I would rather use a straight GL4 oil than risk messing up the synchros with GL5.