Tim,

The short answer is that there is no way I know of to drain the torque 
converter that I know of without removing it- which means dropping out 
the transaxle to get to it.  This is something I have done on a 
weekend, though I can't recommend it for routine preventative 
maintenance.  I now change the ATF every other oil change (about 2.5 
qts), which should keep fairly fresh fluid in the system (by dilution) 
without draining the torque converter.

I started doing this after the auto trans on our '90 Carat failed at 
63K miles.  After three rebuilds at home, I finally figured out that 
the endplay was adjusted incorrectly from the factory- one of the 
rings on the trans.oil pump kept undercutting and eventually breaking. 
I finally ignored the Bentley figure for endplay and bought a shim 
from VW dealer equal to the amount of undercutting.  The shim cost .79 
while the dealer wanted $900 just to diagnose the problem, with 
assurances that it would cost $3500 to fix.  At those prices I figured 
I had nothing to lose trying it myself at home- at the worst I would 
have it towed to a tranny shop with the transmission in a box in the 
back.  Turned out that it wasn't that difficult.  I can now pull the 
tranny out , tear it down, put it back together again and into the van 
in the dark without a Bentley in under 3 hrs!  The last rebuild was 
about 50K miles ago, no signs of trouble. I swapped torque converters 
on the last one though- catastrophic failure/blown clutch packet 
filled the system with metal particles.  I wish now I had kept the old 
TC and had it flushed at a tranny shop (about $15) because the rebuilt 
I installed is of older design and engine runs about 200RPM faster at 
65MPH.

The other recommendation I have is to drop a small magnet into the pan 
while you have it off.  It collects all of the fine metallic particles 
from the fluid due to normal wear- looks like a caterpillar after 10K 
miles. It can't hurt.

I have never seen a bottle of ATF that says only Dexron.  They are all 
labeled DexronIII/Mercon, which is what I use.  What you want to avoid 
are the ones labeled only "ATF" or "Type F", which are NOT compatible.  
Here in San Diego, cold morning starts mean the high 40's, so I don't 
know about the synthetics.  I'd go for the cheaper stuff and change it 
often.

Dan Fuller
'90 Carat
'63 Bug Ragtop
'95 BMW 318i conv.