GL4/GL5

[John Larson]

The specs, and the systems they are intended to be used in, are different.
The rep who told you the GL5 is just improved GL4 is dead wrong.  Audi
specifies 5 for the diff, 4 for the gearbox.  If 5 were just better 4,
they'd use it throughout.  Castrol makes a synthetic GL4, we use it here at
the shop, and it's megabux, well over 3 times our cost for conventional gear
oils.  The VW trans rebuilder we use (German Transaxles of Oregon) won't
even honor their warranty if non-synthetic is used.  80w90 is just fine.
I've used Kendall gear oil (GL5) for nearly 30 years in Porsches and older
VWs, and I'm satisfied there's no conventional gear oil that's better,
Swepco included.  It definitely doesn't work right in the later VW trannys
(Rabbit-on).  Kendall discontinued it's conventional GL4 a couple of years
ago.  HTH, John