The decrease in oil temperature is real.  The temperature sending unit
is in the drain plug and is constantly submerged in oil.  Other evidence
of cooler oil is an increase in oil pressure of about 8 psi at idle (to
10 psi) and about 15 psi highway speeds (3500 rpm, to 45 psi).

Recently someone posted a letter about adjusting the oil level to reduce
the oil temperature in an air cooled engine while they were making a
trip.  I couldn't find the letter but I believe it was this year.

Maybe someone else with an oil temperature and an oil pressure gauge
would like to run a little experiment.  All you need to do is to drain
your oil, replace your filter, and refill with 4 quarts of oil.  Drive
on the interstate for 20 miles at 70 mph or until the oil temperature
stabilizes.  Record the oil temperature and oil pressure at 4000 rpm and
at idle.  Then stop and add a fifth quart of oil and make the return
trip at 70 mph and note the oil temperature and pressure.   I bet you
will then be looking for someway to remove that last quart of oil
because of the dramatic differences.  I suggest you just drain the oil
out and then add about 3.8 quarts of new oil.  Check your dipstick and
adjust the level to  ~1/4 the way up the "Operating Range".  Your engine
will then be nice and clean inside and your oil level will be correct.
You can then post a really interesting letter to the list.

I plan to do this when it gets warmer and I do my next oil change.
Someone else could run a parallel experiment to compare results.

Ed McLean
'86 Westi