Budd is right on recommending Castrol Syntec 5W-50 (synthetic).  Once
your van sits overnight at 0 degrees F or thereabouts, you'll know why
you shouldn't have 20W-50 in there.  On my aircooled, an engine with
larger clearances than waterboxers I'm sure, the battery/starter could
barely turn the engine over and when it did start it first seemed to
run at about 17 rpm.  This is very bad.  Cold-start performance is
probably the best thing about synthetic motor oil.  My van leaked
before I switched to synthetic anyway, and at least with a Vanagon the
smell is behind you.