Motor oil weight
We just purchased this 88 Elite -ford econoline 350 clss C with a 460 cid (7.5L). The previous owner (parts unknown) scribbled on the underside of the hood, among other things, to use 15w40 mobile 1 ...
Forum Discussion
I have never seen a 15W40 Mobil 1 grade. I think a 15W40 such as Delo or Rotella would be just fine. If it were mine and I wanted synthetic, I'd use one of the 5W40 grades of Mobil 1 rated for mixed fleet. I'd be leery of switching it to synthetic if it had not been on synthetic. While it is debatable, I do think there is some slight merit to opinions synthetic causing oil leaks in older high mileage engines which have never seen synthetic. If you're not concerned about that (may not leak or leaks very minor) , synthetic is fine.
