I had (have) an similar problem on my '80 Westy, whose oil light would glow
very faintly at idle when parked, but in motion it would go out.  It turns
out to be a very high resistance short from the parking brake light -- the
lamp goes out when I release the parking brake.  Strange.  Check yours -- I'm
not claiming that this is what your problem is, but it's worth a shot.
Or, if anyone out there knows why releasing the parking brake would make the
warning light go out while the oil pressure is in fact low. let me know so I
don't blow up my engine!

Rusty J.
Ogden, Utah
'80 Westy -- Vanagon Valdiz