I recently purchased a 2001 Infiniti QX4 with 124,000 miles in July from a used car dealership. The car manual recommends 5w30. At the time, I thought it would be best to switch it to synthetic (mainly because I do not know the history of the car and I hear this oil is better?) high mileage oil and change frequently a couple of times. I just started developing a leak with the oil cooler gasket. I realize this could be coincidence or encouraged from the oil; research shows this is a common nissan problem around 120k and it is a simple cheap fix. The issue now is I'm worried about other leaks developing; I have about 4,000 miles in with this oil, should i switch to regular Mobil 1? a conventional oil? I would like to clean out this possibly neglected engine, but now I'm just paranoid! Help!!! EDIT: I also have a 1991 Honda Civic LX that I just purchased for gas purposes. It is extremely low miles for its year (125,000) and calls for 5w30? Ideas for this one? I had planned on using the M1 HM oil, but now I don't know. I would like to keep the oil the same if I can and if it's recommended. I greatly appreciate any feedback for my concerns. Thank You!!!!