What brand of 5W40 do you run? Sound like I'm doing the right thing. From all the info I could find on Shell Rotella it should be a good syn oil. I have ran the 15W40 for years and have had good luck with it. The 5W40 sure made a big difference in the way my truck starts. Before it would just surge and die. Now it start and runs fine. My 99 has a 98 build date. I sure like it as I have had it for going on two years now. It has a little over 150K and runs like a new truck.
99 Red Stroker
Ron