Why is my vw van leaking oil from the shaft seal benind pulley on crankshaft?  

Year? A little more info would be helpful.  

It's a 1983 aircooled VW Westfalia.  

It's a 1983 vw westfalia camper Vanagon.  

Well, it may sound silly, but my guess would be the seal went bad. They will eventuall wear out or dry rot and crack.  

Sounds about right. You want to be especially careful about that stuff on air-cooled engines, since they're essentially oil-cooled. If you lose too much oil on an air-cooled engine, that could be real trouble.

Another option is to upgrade to a water-cooled Vanagon engine, eventually.  

Volkswagons don't have a rubber seal like other motors ,it just has a few flat metal washers that are real thin. The engine vacum is suppose to suck them towards the motor and create a seal against a smooth metal face. ,the motor loses half a litre of oil every two tanks of gas at present. The problem has been going on for a long time. I had the motor rebuilt 20,000 miles ago and it still has the same problem ,It's driving me nuts!!  

I copied a blow up of the motor crank assemble and there are rubbers seals after the metal shims and a rubber seal at the pulleyend too . May be it is a seal leaking. It's almost impossible to get at it with the engine in the car.  

Yeah, my father-in-law works on VWs, and a lot of times, especially on the vans and microbuses, he has to drop the engines to do work on them. Might be your only choice.