A couple of articles that might interest you:

http://www.bostonengine.com/articles/low-oil-pressure.htm

and see www.vanagon.com --> INFO --> Articles and Essays and scroll down
to Maintenance --- > Oil Pressure Problems

Cheers,
Doug

"Mike D." <md03@XOCHI.COM>
Sent by: "Vanagon Mailing List" <vanagon@GERRY.VANAGON.COM>
01/08/2004 02:22 PM
Please respond to "Mike D."

        To:     vanagon@GERRY.VANAGON.COM
        cc:

        Subject:        Re: what causes low oil pressure

>Date:    Wed, 7 Jan 2004 23:41:47 -0800
>From:    Ben S <phlogiston@ISPWEST.COM>
>Subject: what causes low oil pressure
>
>[...]what causes the oil pressure to get low
>enough to trigger the light?  assuming that the oil was getting too hot
>and thin from all the climbing, why did the pressure go up so quickly
>when i let off the gas?  the oil certainly can't cool down that quickly,
>and i would expect that a decrease in engine speed would decrease the
>oil pressure.

My van has a very similar problem, though somewhat reversed from yours.

Key on, engine off: oil light ON
At idle when cold : oil light OFF
At idle when warm : oil light ON
At 3500 RPM accelerating hard at freeway speed when very warm :
flickers just a bit.

I too think this is weird : why would my oil light come on at 3500
rpm under heavy load and go OFF when I let off the gas (but the RPMs
are still 3000+)???

Perhaps under heavy load we are getting cylinder blowby, which is
pressurizing the crankcase and this is doing something weird to the
oil pressure (or sensors)?

What else happens at full throttle?   Low vacuum in the intake?
Can't see how that would affect oil pressure (or oil pressure
readings)...

very odd.