I run the Series 3000 5w- 30 in mine and noticed no change in the pressuers except here in the winter. The oil pressure is more consistent the orignal 15W-40.
I installed the bypass filter system 2 days ago and there is a small drop in pressure. But that is because there is 2 foot and 2 new filters that the oil get pushed through.
I am no diesel tech, but the tech at my GMC diesel told me that the oil pressure was sort of irrelevant to the engine in this way. Each of our rods has an oil slinger on them that throws oil into the cylinder every rotation, the oil pump basically just pumps oil through the filters then makes sure there is oil sprayed on the rods. But I could have misunderstood in my mind as he was describing it. It sort of made sense to me, but he said low oil pressure to him, just meant that the lubricant was being more easily pumped. So I do not worry about it at all. I know that the oil is the best on the market for lubrication and additives and I now have 2 top of the line filters doing their jobs and it all is efficient and trouble free.
Just FYI, I also put 2 heat sinks on the filters and strap on magnets to further aid in cooling and also magnetizing the filters to keep all metal dust in the filters.
I am also looking at the new formulation from Amsoil in the 5W-40 for the 2007 and newer trucks, it is backwards compatible and has a few other features that may help with this new fuel. I do not for sure on it, as I am and always have been really skeptical about this stuff, but from 2002 until now I have been a 100% amsoil fluids guy and the stuff works and has saved me $$$$ I can not even calculate.
I have checked the oil level several times and looked for leaks everything seems fine. At idle the pressure is exactly the same as before with the old oil. Just when I am running down the road is it different. Did anyone else experience this with Amsoil or is this just something that happens as the truck breaks in? 
