Help me choose the right oil for my Jeep Grand Cherokee. I have a 2002 model equipped with a 4 liter engine. The mileage is about 136,000 miles. I use the car mostly in urban conditions, periodically going out with my family for a picnic in the woods. The Jeep does not experience heavy loads. I would like to choose an oil that will prolong the life of the engine - the price does not matter. 