I'm probably going to be buying a 2000 Mercury Mountaineer from a co-worker sometime this week, and I want to know which oil to use in it. It has the 5.0 V8 and 58,000 miles on the clock. It has been dealer maintained ever since he has owned it. Would the dealer use 5W-20 in this motor or 5W-30? I'm planning on using Mobil 1, whichever grade it uses, because it is readily available.