I have had this same problem myself in my 87 GL.  What it turned out to
be on mine, was that the temperature of the engine and the temperature
outside would make the oil at freeway speeds get just thin enough that
slowing down would trip the sensor.  With mine, if I stopped and let it
sit for 60 seconds or more, it would go away.  When I changed my oil
next, I used the 20W50 and have not had this happen again.  Hope this is
helpful.
Phil
> -----Original Message-----
> From: Jonathan Clay [SMTP:Jonyqest22@AOL.COM]
> Sent: Thursday, October 08, 1998 2:10 PM
> To:   Vanagon@VANAGON.COM
> Subject:      oil pressure light
>
> Hey there everyone,
> I couldn't access the archives for some reason and I'm a bit impacient
> right
> now so here it goes.
> Once my van gets goin' (i.e: warms up) on highway rides she's great no
> problem.  However once I exit the highway the oil sensor light goes on
> and
> stays on until I either downshift or speed up to get the rpm's going.
> This
> usually happens around 27000 or below.  It's not oil amount, i've
> checked it
> several time to make sure.  Does this sound like an oil pressure
> problem or
> maybe an oil temperature problem or......other.   This usually happens
> on
> average warm days up here in VT and since they seem to be long gone
> she
> doesn't do as often but she still does it and
> .........damb...........so
> annoying I've got to get this fixed ...soon
> any suggestions/comment will be helpful as always
> sorry I had to wright to everyone directly.