One thing to watch in todays oil market is the phosphorus content of the oil. Many synthetics do not have it or a very low content. The 4.0 is an old school flat tappet engine that requires those types of additives. Most lifters on the newer engines, like 90 and newer have roller cams/lifters in them and don't require those additives. The break in oils have extra amounts of it in them to help break in the camshafts, which on a flat tappet engine have to be work hardened to keep from wearing out prematurely.