1] Poked around the bottom of the engine.  Pulled the pressure valve and
the relief valve.  Wouldn't you know?  The relief valve plunger is stuck. 
While the ol' screwdriver trick works to get the plunger out of the hole,
I can't message the plunger or clean the hole sufficiently so as to stop
the plunger from getting stuck each time I try to put it back in the case. 
It should just "drop out" after the plug and spring are removed, right? No
similar problem with the pressure valve.  Hmmm.  I guess I'll go to Austin
VeeDub on Monday and get a "newer" style plunger that has a flanged end. 
And some advice. 

 2] I think the sender is o.k.  The idiot light comes on
strong when the key is turned, goes out as soon as the engine starts, and
acts up only during the conditions I described earlier.  I am encouraged
about discovering this sticky plunger-- maybe an easy & cheap fix. 

3] Per Jamie's suggestion, I looked again -but found no oil leaks. 

 Nick McRee
'66 Westy (sick) '73 Thing (well)