i changed my oil yesterday and i noticed a big change in idle quality even though the oil had only 3k mi on it. and the oil press went up on the gauge to 'm' in 'normal'. i know that the oil weight changes with mileage from the hpop being hard on the oil. i have heard the term oil shearing, and that the oil goes from 15w-40 to 30 over time. what can be done to prevent this. and can i run a straight weight to help, or a additive? all i know is that there was a huge change in performance and idle quality after 3k mi and 2k of that being towing. what can i do?