I have a 2000 Jeep Grand Cherokee with a 4.0 liter inline 6 cylinder engine that has ran Mobil 1 full synthetic since the 1st oil change new. . It had 265,000 miles before we had a cracked head and broken piston. Engine has been rebuilt and we were told by the mechanic to not put synthetic back in until at least 60,000 miles. This makes no sense to me as we got 265,000 miles running full synthetic after the first oil change new.
Thoughts?

 

You can run synthetic (or what the oil makers are calling synthetic) with no problems. Your mechanic doesn't know what he's talking about.

 

What rgr said. If its good enough for brand new arctic submarines its good enough for your rig.