I use what the owners manual recommends as to weight/viscosity. Unless it calls for synthetic I use conventional oil, too many think synthetic conveys some magical properties that it doesn't. I have a 1998 dodge diesel with 377,000 miles that has had nothing but conventional shell rotella 15W-40, it runs and pulls like the day I bought it new. I have a 2006 diesel Jeep Liberty that does require synthetic so it gets shell rotella synthetic 5W-40. My 2006 Massey 471 gets Rotella 15W-40 conventional, same as the truck. As you can tell I like the shell rotella series of oils, I've had great service out of them. I don't have access to a MF1643 owner's manual but I'm a bit suprised it calls for 10W-30, I'd expect it to recommend 15W-40 with the 10W-30 as optional for a colder climate, which Alabama isn't. When in doubt I lean toward the heavier weight oil, I'm more worried about protecting my engine in the Mississippi heat than I am about saving fuel. Whatever you run make sure it's a C (for compression ignition) rated oil. I suspect your manual calls for a minimum of CH rating so make sure it's at least what's recommended. Mobil 1 10W-30 is only rated to CF from their website so it's probably not going to meet the requirements. Stick with oils from a major manufacturer with a good reputation, don't fall for the hyped sales pitch and pyramid marketing schemes of companies like Amsoil.
