I bought it without knowing what oil was used. I used Pennzoil Ultra Platinum and Mobil 1 and both oils showed high wear metals. I started getting concerned thinking the previous owner neglected it. Then someone recommended Quaker State Full Synthetic. I was skeptical at first, but the results gradually improved and now I'm getting good results.
From what I hear, and this is just rumor, Mobil isn't as good as they used to be. It's still good oil, but they have been cutting costs and getting by with their reputation and brand recognition.