Jeep 4.0 motors Do like the Higher levels of Zinc/Phospherous found in Diesel oils, High mileage oils, and thicker weight oils(10w-40 and thicker). I have 4 Jeeps in use now with the 4.0, and I really do believe they like a thicker oil. The engine design is Old School, the types of oil used in these engines when the were designed were the thicker weights. Thats not to say that these engines couldnt survive on a thinner weight such as 5w-30, they can survive just fine with that. I just think with Hard use, these motors live longer with a good, thick 10w-30 or 10w-40. Quaker State is my New favorite for most any year or condition Jeep 4.0.