Always an interesting topic that never fails to promote discussion.

Here are a couple of links I've found useful:

http://www.vtr.org/maintain/oil-overview.html
http://www.vtr.org/maintain/lubricants-redline.html

This is from the Vintage Triumph list, and there is other good info here for
us "vintage" Vanagon owners.

What I've found with the WBX (and the VW air-cooled too) is that the bearing
clearances open up as the engine heats up, causing the oil pressure to
decrease (adding an oil pressure gauge is very enlightening!). So, the
physically thicker the oil at high temps, the higher your oil pressure will
be.

Here in Seattle, a temperate climate, I use 20w/50 year 'round.  But if I
were in Phoenix, I'd probably use a straight 50w in the summer and a 40w in
the winter.

Multi vis oils use polymers to extend the viscosity, and these can break
down causing ring and valve problems.  That's why they aren't recommended
for use in two cycle engines.

Probably the best solution is synthetic oil, since the higher percentage of
uniform long chain molecules in the base stock requires fewer additives to
attain a reasonable viscosity spread.

Stuart MacMillan
'84 Westy w/2.1
and lots of MGBs