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Your 4.0 requires a 5w-30 oil. If I were looking to maximize the life expectancy of the engine, I would be running an SL oil, if you can find the old stock (current production is SM), and I would be adding a half bottle of Valvoline Synpower oil additive. It is loaded with ZDDP which is a multifunction additive. Your 4.0 is a high stress engine and can use all the help it can get. Following this regimen, your engine should coast pass the next 100,000 miles and far into the next 100,000.