As I see it...the VW personages tried to improve on the shifting whilst
using Dino oil and not synthetic...With Redline or any good quality
modern synthetic, the point is moot due to the thinner viscosity  of
modern oils...When VW was trying to "modify" the shifting it may have
been to improve the old stuff that was truly 90 wt and not the 75 or so
weight oils we now use..
 I know many shops that just put any old thing in there and fill till it
dribbles out the fill hole...never have I had a customer with a problem
that I am aware of from filling it that way. I firmly believe that just
changing it once in a while is the key to longevity...as the trans wears,
a synthetic may help the shifting more than a fresh rebuit trans. I
haven't put anything other that Redline in for years, so can't vouch for
how most shift with plain old 90 wt in 'em.
 Just change it every 30K and use synthetic if available (esp in colder
climates)
Daryl of AA Transaxle
Duvall, WA.(Seattle area)
(425) 788-4070
1-877-377-0773 toll free
http://www.aatransaxle.com (web site)