I just replaced the entire front end of my 04 5.4 last month and am still seeing the engine stalling, moreso when I back into a parking spot after the engine has come up to operating temperature. Since the phasers and everything are good (assuming I didn't get bad parts, that is) my thought is low oil pressure. I've heard 20w-40 and now 15w-40. What is the best weight to use?? I don't want to add too much density. And does anyone have any first-hand experience to share about using heavier oil over time?