Hi Paul.  I was just asking my dealer about this 2 days ago.  They 
didn't think it was necessary to change the viscous coupling.  They 
use Pennzoil 75w-90 GL5 synthetic in the front diff, and transaxle.

Not too sure about this, though.  I checked in Bently, and it says use 
GL4.  SO, more research is required.

> 
> The reason I ask is that after hearing such glowing reports, I was going to
> have the tranny drained and refilled with Swepco 201.
I, too was asking about the Swepco 201 oil.  The parts guys uses it 
in his motorcycles.  He says it's good stuff.  BUT, didn't confirm 
about it's use in the Syncro.
 
> Should I proceed with this or not?

I'd do some more research.

> Also, (showing that I haven't had time to read through my Bently), is the
> transmission gearbox directly connected to the rear differential unit?
> 
> If so, do they share fluids?

Yes.  It is one unit.  Do you have the diff lock??  It is because of 
the diff lock that the parts guy didn't go all out  and recommend the 
Swepco oil.  He wasn't sure.

> If not, what sort of lube goes in that differential unit, and is it the
> same lube that goes in the front unit?
> 
> And if I'm going to be super-maintenance anal (not a bad idea knowing how
> much these parts cost), should these fluids be drained and replaced at any
> sort of regular interval?

Please keep me posted if you find out anything.  I am also very 
interested.

TIA
Harry.
------------------------------------------------------
Harry Yates      Internet: hyates@bcit.bc.ca  Burnaby, BC
'90 Syncro Westfalia
'87 Golf Diesel