[Diesel_Dave]

Earlier this year I switched my diesel truck over from Shell Rotella 15W-40 conventional to Amsoil Series 3000 HDD 5W-30 synthetic. It's one of the few mods on my truck that's had an immediate, noticable impact. I was expecting someting on the order of a 3-5% FE improvement, but saw a 5-10% improvement.

I think part of the reason for that is the fact that I run such a large engine and I suspect during normal comminuting my oil pan hardly ever gets up to "full" operating temperature--I'm usually almost halfway there before my coolant temps get all the way up and the oil temp will lag the coolant temp.

However, I do think it's more about the viscousity than whether it's "synthetic" or not--now days the line between synthetic and non-synthetic is getting blurry anyways.