I switched over to Castrol 5w-50 synthetic oil this past fall in my 86
Syncro with 146K.  Prior to this, I had been running Castrol 20w-50.  Since
then it has run perfectly for 6000 miles, including an 1800 mile road trip
(towing another van for 900 miles), with no difference in oil consumption.
Previously, and now, it consumes about 1 pint of oil every 1500-2000 miles.

My advice is to shift to synthetic.  The ease of cold weather starts is what
I notice most.

Budd Premack
86 Syncro, 84 GL, 73 Super
Minneapolis, MN (Land of Frozen Waters)