I have a 2000 Ford Focus SE with the 2.0 liter Z-Tec engine with approx. 70,000 mile on the clock. It's been using about 1 quart of oil every 500-750 miles and it's not blowing blue smoke and no oil being forced out of the valve covers. I've been using 5w30 Valvoline for the last 2 years, but should I try using a heavier weight oil in the Z-Tec?