I have a 2006 F-250 with a 6.0 diesel engine. I have been using Motorcraft 15w40 since day one. I am considering switching to Amsoil 15w40 full synthetic.You can sign up for a prefered customer for $20.00 a year and order the oil directly for $20.00 a gallon. I currently pay $9.89 for a 5 quart jug from WalMart. Is this really going to keep my engine runing better, or should I stick with the Motorcraft?