i use the royal purple 10w30 full synthetic and i love it
ever since i used it i can feel the motor runnin smoother and even felt a little bit of a power gain
only fall back to runnin full synthetic oil is u AWLAYS have to for the rest of ur jeeps life u CANNOT switch back to conventional oil

 

I usually switch between 10w40 in the summer and 10w30 in the winter here in the north east. I think it really depends on the weather you typically drive in. If it gets pretty hot out and you run the a/c hard like i do, i personally like 40 weight as the oil pressure does not drop so much at idle and sitting in traffic. In the winter i go back to the 30 weight as it makes starting a little easier.

As for brand and type, i just use regular dino oil. Usually between Penzoil or Exxon.. whichever my local auto place has.

 

you might want to consider using an oversize filter eg. NAPA 1085 is the normal filter but a NAPA 1515(oversize) will also fit. you either get more surface area to filter the oil or you can add more than 6 qts. if you desire.

 

Absolute myth......just one of many.

 

I run 5qts of Lucas 10w-30(High Zinc) and a bottle of their Oil Stabilizer, seems to work Great, when i first bought this XJ, it had some build up when i took off the Valve cover, this seemed to clean it up real nice.http://www.lucasoiloffroad.com/lucas...yn%2010W30.pdf

 

X2

 

I use royal purple or rotella.
Rotella has all. The. Good. Stuff our. Engine needs

 

I use Rotella T6 5W-40. You really need to run oils (specifically diesel oils) with a ZDDP package, or add it yourself. Flat tappet camshaft-equipped engines, like the 4.0, need zinc and phosphorus in their oils for improved cam life.