This is a correction to my earlier posting.  (Thanks to Gerry for pointing
out my oversight.)  The regulation VW block heater bolts onto the bottom of
the engine block, not to the oil pan as I had earlier stated.  The part #
is ZVP289104 (typo on earlier post) @ $ 43.80.  This info came from
Westside VW in Minneapolis, MN, (612) 377-4100.  The cord that came with my
unit was short, so you may be well advised to lengthen it prior to
installation.
With a synchro, you can't bolt this heater on, so you must obtain an
aftermarket magnetic-type heater of the proper physical size.  This won't
magneticaly adhere when there is no steel, so glue it on carefully.  Be
sure to work in a warm enough environment for proper adhesion to occur.
(Remember, we are talking about winter care, and not all of us get their
preparations done in a timely fashion.)  The heater on my synchro also is
wrapped in duct tape as a precaution against adhesion failure.
My personal tests on an 84 Vanagon and 86 Synchro were completely
successful, even at -25 F, sitting outside in the wind.  I think a heater
is a good precaution, and good preventative maintenance so that you reduce
the amount of revs put on a cold oil-starved engine at initial cold start.
It also makes it much easier for a cold battery to spin your engine to
life.