Dear Doctor: I decided to switch to a high-mileage oil due to the age of our 2005 Buick Rendezvous that has over 61,000 miles on it. But my service adviser said this is unnecessary since I'm already using synthetic oil. He also stated that I couldn't go back to it, after using blended oil. My 2003 Caddy has 127,000 miles, so I would be using the same type in both cars. What is your advice? - Daryl
Dear Daryl: It is old-school thinking that you cannot switch back to conventional, synthetic blend or full-synthetic oil at any oil change interval. Full-synthetic oil does not thicken when cold, or cake up and burn when hot, and offers superior lubrication. It will not cause oil leaks but will seep through any existing oil seal leaks that are present. It is critical to engine operation to use the correct oil in newer vehicles. The use of the wrong oil will cause internal engine major failures. Oil must also be changed at recommended intervals or sooner, never later. I personally prefer a good-quality full-synthetic oil, if the oil specifications meet the carmaker's requirements.