I have a 99 nissan altima. has 78,000 miles on it and its mostly highway. the engine seems to be leak free from oil. I was thinking of switching to mobile 1 so i can change it every 3 months(7,000 miles) instead of a month in a half. the other oils like amsoil, redline etc... seem to be better but i want an oil i can buy at walmart or at a parts store readily available. one of my ?'s is that if switching to synthetic that it may open up any leaks that dino fuel has sealed in the engine. is that true? also when switching to synthetic should i use an engine cleaner? will an engine cleaner clog up my oil pump with the sediment left over? what kind of engine cleaner is good? any suggestions are appreciated.