I have been running Castrol Syntec (full synthetic) 5w-50 in my 86 Syncro
since last year.  Temperatures have ranged from -20 to 90 F with no problems
at all.  The noticeable benefit has been much improved cold starting and
running in the winter.  Also, there has been virtually no oil leakage or
burning with the synthetic, which is the same as the regular Castrol 20w-50
that I previously used.

Budd Premack
86 Syncro, 84 GL(for sale), 73 Super
Minneapolis, MN (Land of Sky Blue Waters)