Dear Doctor: I am switching to a high-mileage synthetic blend 5-30 oil in my 1999 Ford F-150 with about 145,000 miles. When I need to add a quart of oil between changes can I safely use regular 5-30 oil? I have a few quarts of regular that I hate to waste if they can be used in a limited amount with the synthetic blend. Paul
Dear Paul: The rule of thought about mixing oil is old school. Adding a different brand, viscosity, type regular, synthetic or synthetic blend will not cause any harm on vehicles with high mileage (70,000 miles and higher). On vehicles still under factory warranty, you do have to stay with the factory recommendation for oil viscosity and type. High-mileage oil does have a different makeup with an additive package for engines with wear. I do not recommend the use of high-mileage oil on engines with less than 70,000 miles, unless the engine has internal problems, such as leaks and burns oil.