re: Ot mechanics: gas mileage issue

First off, if you changed to a "high mileage" oil for no other reason than you thought 129k is high mileage, you done fricked up. High mileage oil just has extra seal swelling additives that might help with slight leaks around valves and front or rear main seals. You should notice that the API Service symbol on the bottle probably does not say "energy conserving" or "resource conserving" too. That's because these oils use additive packages that are outside the API service spec, usually more EP, VII's, and detergents. All good stuff if you care more about reducing engine wear than peak fuel economy and catalytic converter life. Problem is the seal conditioners will expand the seals a little and they will wear in to that condition, meaning you are stuck using a high mileage oil to maintain the seals in that new state. That's why you should not use such an oil unless you suspect a problem with oil control that could be helped using it. All that said, 50 miles in one tank means nothing if you changed fuel brands or a number of other factors. Keep checking for another few months.