The '96 cherokee with the 4.0 engine I just bought has 194,000 miles on it. The owner had the oil changed every 5-6000 miles and they are mostly highway miles. (he was a salesman that drove around 300 miles away weekly) He might have used synthetic oil, but I don't know for sure. Either way, it runs perfectly. What oil do you guys think I should use? I'm going to go with dino for now. I was thinking penzoil long life 10w40.