I rechecked the gauge's accuracy and found that, this time, it is dead
on. Boiled a full kettle of water on the Westy stove and dipped the
dipstick sensor in it: gauge showed about 210-215F. Of course, this
does not yet prove that it will also correctly measure higher
temperatures, but the 285F reading this past weekend becomes even more
disconcerting. I now come to suspect that the oil cooler is not
working at all and my plan is to check the oil pressure relief valve. 
As you know, its function is to let oil bypass the cooler when it is
cold and thick and my guess is that it is stuck. I really hope that's
the problem, since this would be a relatively easy fix. Could 285
degrees have harmed the oil (Mobil 1) so that it should replaced?

(Btw, I just read in Muir that the ideal oil temp is between 180 and
220F and that temps above 225F should be avoided. Dunno if they knew
about synthetic oils when they wrote this, however.)

I'd like to thank John Anderson, Paul (pwies@loyola), Unca' Joel,
BusBoy Jim, and Dennis Haynes for their kind replies.

Sami