I am new to the diesel world and was wondering when I do my first oil change, what oil should I go with? Should I replace it with the 15w-40 they put in from the factory, or should I change to 5w-30 from Amsoil? Also do I need to replace the external Transmission filter at that time? Also what is your opinion on the oil filtration system from Amsoil? And also do I need to use any fuel additives? Thanks for any assistance you guys/gals provide me with!!!