The Mobil 1 prefils are indeed 0W-40. Only some dealerships have this oil. I wish I could get my hands on some.
I just recently used the Valvoline SynPower 5W-40 and found I vastly prefer Mobil 1 15W-50. The Valvoline causes the oil pressure to be a hair low at idle for some reason. I'm not sure that's anything bad, but I don't necessarily like it.
1988 Mercedes-Benz 260E