Hiro, What year was your Jeep? Up to about 95 or 96 they suffered from a steep backside ramp on the cam. The valves tended to slam closed causing a lot of valve train noise. It seems reasonable to me that a motor oil that is nearly the viscosity of an SAE 40 at operating temp would be well suited for the 4.0. Was the noise worse at startup or after the engine was hot?
I had a 1993 and a 1997. The '97 simply made valvetrain noise at idle, more when warm then cold IIRC. I purchased it with 60K miles and used Mobil 1 10W30 and either a K&N, Mobil or PureOne filter every 4000 miles. I sold it with 120K on it and no leaks or consumption of any kind. I had tried 0W-30 and 5W-30 in the winter and decided it simply made too much noise, and tried 15W-50 once in the summer and decided it wasn't necessary.
"What is a 'heavier barrier additive package'?"
Elevated levels of zinc, phosphorous, molybdenum and/or borate esters.
The oils which most consistently minimize piston slap tend to have higher levels of these additives ... with molybdenum seeming to be especially effective.
--- Bror Jace