>>
>> Here's what I've done: took the van to VW to have the pressure
>> measured. They said it was low. I don't have the numbers with me but
>> I seem to remember 4psi at idle.  Changed oil from Mobil 1 10W-40
>> to Castrol 20w-50 to straight 40 weight. Each change helped in that
>> it took longer for the warning to to start. Have always used VW oil
>> filters. Had oil pressure sensors replaced. Replaced the oil pressure
>> regulator piston and spring, had the oil pump replaced.
>>
>> Is there anything I'm missing?
>>
>  maybe ..... are you over heating?

I have been following this tread, wondering what could be wrong, when all
the sudden something struck me.  My friends Jetta was getting 8 psi at warm
idle, and he was nervous.  We took the whole damn engine apart and
everything looked fine.  Tight bearings and like new oil pump.  We finally
discovered the pickup tube was clogged with debris.  I would check this,
although this is extremely difficult to do on this type motor.  Also check
the connection where the pickup goes into the oil pump.  I have personally
never opened a WB, but have experience with T4's, and know the pickup can
come loose from the pump.
good luck, PSD.