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?

 

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?

 

Are you using non detergent oil as break in oil? I have always run the first 1,000 miles on non detergent oil and did my first oil change. I then went to the oil I was going to use and started using it. Changed at 3,000 to 3,500 with normal oil; 5,000 with synthetic and went on down the road. Current vehicles have 167+ and 100+ on them and the oil is not dirty when changed.

 

FYI, the Grand Cherokee section is here. This part of the forum is for the regular Cherokee.

Find a new mechanic. Yours is an idiot.

That's because you have good sense!

 

I moved this thread to Grand Cherokee tech.

Your mechanic is indeed an idiot. All sorts of new vehicles come from the factory with synthetic oil.

Trust your own intuition.

 

Most synthetic oils today are not true synthetic, but ultra-refined petroleum oil. Ashland (Valvoline) runs it thru the cracking tower 4 times to purify it. It's still petroleum, not chemically built molecules. It's as stable as they can make it as far as breaking down under heat.

I used to use full synthetic in my 4.0 until it started using oil (as in burning it). Now I'm using conventional oil and cut my oil burning down considerably. I have ZERO, ZIP, NADA sludge in my engine because 3 times a week, it's running continuously for at least 3 hours while delivering newspapers.

Personally, I think synthetic oil is all hype for normal vehicles unless they are run at high temperatures with heavy loads all the time. In all the years I've been driving, I've never had an oil related engine failure using conventional oil. I DID have a fresh oil fill turn black in an air-cooled VW beetle on a trip from FL to NY though.

 

Perfectly fine to run synthetic after the rebuild.