Randy-
My '89 westy did the same thing.  The oil light used to come on once in a
great while after extended highway cruising only when the outside temp. is
high and I had the AC on. Now that I replaced the thermostat it comes on
more often and without regard to outside temp,etc.  My only thought is that
it just wasn't reaching full operating temperature before which now it does
quickly(I can see that on the temp guage).  Funny thing is I have an oil
pressure guage too and it seems to idle at about the same pressure as it
used to (just below 10psi like you said). I assume you've taken the usuall
steps of correct weight oil (20/50), correct filter, etc. I guess we'll have
to face facts that our main bearings aint what they used to be.
-Michael
Augusta, Mo.