Petroleum oils will harden seals and gaskets over time. Good grade synthetics will keep them softer, but can soften hardened seals and possibly cause leaks if you switch from petroleum to synthetics. 50,000 miles is a basic break point for car and truck motors where you might run into problems if you were running one type then switch. Government oil grades type 2, type 3 are petroleum grades, type 4 can be petroleum and synthetic mixes, and the best grade is PAO (Polyalphaolefin oligomers) synthetics. The type 4 group is the problem area because mfgs. can label their oil bottles as synthetics, but lower quality and mixed with petroleum without giving the mixing ratios. Mobil 1 I have been told has been an APO oil for years and I've been running it for about 17 years in my cars and trucks and doing 15,000 mile oil changes. No problems so far. My BMW I had recommended 15,000 mile oil changes done free at the dealers under their maintenance with BMW synthetic. In the manual, if you couldn't find BMW, they recommended Mobil 1. I've been in the oil and grease business for 17 years, but for legal reasons the above info could be incorrect or out of date.