FWIW, I had been running Mobil 1 in the engine since  5k after its
rebirth, but I've had several instances where the Oring seal  at the
oil cooler was leaking.  It was doing it again last winter and into
the spring, until I changed oil and returned to Castol High mileage
10/40 oil.  The leak, if not totally stopped, is much, much less for sure.

Since the O ring seal is black and presumably not Viton, and since
full synthetics love/demand Viton seals, could this be a coincidence,
or something more??

Does such a thing as a Viton Oil cooler seal exist??  An argument
could be made that the oil filter seal is not Viton and normally
doesn't leak, but I am at a loss to explain this in another logical fashion.

TIA

DM&FS