I had this very same disconcerting problem occur this past weekend in
my '87 waterboxer (for now).  No light for the first 20 minutes of operation,
then a flicker and a buzz when decelerating (came on in the 2800-3000 rpm
range)  Pull over, check oil, wait 20 minutes or so, repeat (I did this from
PA to CT, thank you very much.)  Oil level was fine, oil was cheap Esso 10W30
with a gennie VW filter, three weeks and 500 miles since I changed it.  I
brought it to a Vanagon-friendly mechanic on Wednesday and picked it up this
morning.  He had checked the oil pressure and found it to be within normal
range for a used engine, but found the oil to be very thin.  Here is what I
found to be of interest and concern: he tested the oil (he has all sorts of
cool equipment) and found it to be heavily contaminated with hydrocarbons.
He assumed that the oil was old and changed it, putting in another VW filter
and 20w50 this time (normally, 10w30 would seem appropriate for a New England
winter)  Anyway, the flicker is gone, but I'm left wondering, how is gasoline
getting into and thinning the oil so much and so quickly?  Is the FI running
too rich?  Fuel getting past the rings?  It passed emissions in January and
has been running well, so I'm somewhat baffled by this.

P.S. engine for sale much cheaper now, in light of recent developments...the
heads still don't leak!