I have a 1979 Cadillac Coupe DeVille with 9,000 miles on it (not a typo!). It has a 425c.i V8 carb/gas engine (flat tappett). I'm still running a catalytic converter on the car ('Vette style cat). Right now I'm using Shell Rotella T 5w40 syn (CI-4). Since this isn't a daily driver, I'm only putting about 2,000 miles a year on the car. Should I stay with a syn, and change once a year, or go dino and change twice a year? Also, I don't want too much zddp to hurt the cat, but want enough to prevent tappett/cam wear. Winter temps in my area (NC) rarely go below 25 degrees (car is also garaged and usually not taken out in extreme cold weather). Not sure I want to keep using diesel oil, so what are some choices for my situation?