I use only Mobil 1 Fully Synthetic and change it every 10,000 miles in both cars.

2003 Pontiac Grand Prix with 122,000

1999 Buick Park Avenue with 200,000

I've had Buick 3.8's in the past go close to 300,000 and never had a failure. The only reason I've never seen more than that is because I sold them or the wife wrecked them!

Both cars get fully warmed up and see plenty of highway miles to burn off contaminants.

I tried going to a lighter 5w30 over the recommended 10w30 for better mileage but they started burning it.