"""My et is that the oil is overfilled causing it to get too hot and foam"""

Good point Dennis, and that is all "too much oil" will do to an engine,
unless you put 6-7 liters:-))))... (even if Porsche racing as to rise the
level on theirs boxer because wile racing, oil was moving too much on each
side and the pump was having troubble picking up oil at the bottom of the case)

The pump create pressure, Sender(S)-(86-91) read that pressure (well, when
they work!) and they don't know how much oil is IN the engine, they just
need some, sadly, those senders will only tell you that your...out (or
almost) of oil, and, it's too late.... (a bit less on 2.1L but on half of
ther van i see, the wire is grounded on the block / not connected to the
sender), for those that don't understand what the senders do... they are
hooked to the oil red light / buzzer. A oil pressure gauge is way more
precise and will react way faster (but again, are you looking a those all
the time!!!!).
http://www.benplace.com/79beige/79beige44.jpg

The Boxer, mainly because they are old have a problem creating pressure,
that is why we use a thicker oil, like Dennis said, 10W40-15W40 is fine for
colder weather. If you have a VDO oil pressure gauge on board, you would be
shock to see that oil pressure dosn't reach a standard volume with some 5w30.

Senders were replace with more tolerant one over the years, especially the
lower one (between pushrod tube-all years).

I did a section on that a few years ago...
http://www.benplace.com/oil_pressure_sender.htm

I'm very happy with Penzoil! (Castrol is fine also)

Regards, Ben