Some cars, like Astra Redline, spec a single MTF change at 6000mi, then no others.
Be aware, new VAG MTF is VERY thin at 6cSt, thinner than ATF. Tuner shops still dump in 75w-90 GL-5 GEAR OIL. Going up a little is not a bad idea, it's quieter and handles more power easier. With this said, Pennzoil Synchromesh is really the best of the bunch at 10cSt. Redline MTL, the thinnest blend, is also very good.
Guys in Canada used Dexron III Mobil 1 ATF for a long time for extreme cold. Dex 3 or 6 is correct as far as not being "friction modified", and will let the synchros grab. That's why GL-5 gear oil works poorly, it's too slick for synchro engagement. Same for ATFs like ATF+4 or Mercon V.
I'll look the thread over better and chime in again if I can help. PZ Synchromesh was like butter in every trans I used it.