Possible TR-4A purchase: oil pressure question

[John Cowan <jfcowan@earthlink.net>]

   Checked the compression on the TR-4A I'm considering for purchase.  All
cylinders checked out above 150 PSI.
   Oil pressure is between 60 and 70 PSI at speed, but falls to 10 PSI at
hot idle.  (My TR-3A shows from just under 50 up to 60 PSI at speed and
varies between 15 and 50 at idle.)
   What might cause this and do you see it as cause for concern?
  Thanks.
John Cowan