Synthetic gear oil in OD gearbox

[Joe Curry <Spitlist@gte.net>]

I recently changed the gear oil in my Street Spit and couldn't find any
GL4, so I decided to try the Synthetic stuff since it is not supposed to
react with the brass parts like GL5 is reported to do.

Afterwards, I noticed that when I downshift to Second, it almost always
grinds.  Since it never did prior to the change, I am wondering if the
synthetic stuff is too slick to allow the synchro rings to grab
properly.  

Any experience or opinions on the subject?  And by the way, the gearbox
is a MkIV (3-rail) full synchro with D-Type OD.

Thanks,
Joe (C)