I have a 2003 Corolla with 125,000 miles. I have owned this car for four years and have doubled its mileage. In that time I have used exclusively mobil 1 5w-30 and pureone filters every 5 to 6,000 miles. I feel that this may be overkill. I know that this is a good engine but is also prone to sludging. When a friend (mechanic) adjusted the engine valves about 20,000 miles ago he commented that the engine looked as clean under the vavle cover as a car with 10,000 miles, and that is after his 30 years of working on cars. Would you extend the drain interval. I like to play it safe but wonder if I am being too conservative with my oil changes here.