Really like my amsoil (my preferred choice) in the colder climate that I live in. I use it in my diesel truck and my jeeps. My 01 Cummins (old truck) would start no problem, no plugged in, to temps down to 47 below air temp. That was my record anyway. The engine cranked over just fine. Same in my old 2000 XJ. Start anytime. Both vehicles ran perfect before I switched them over so I guess I have nothing to say about that. My Mjs leak oil so I do not run synthetic in them because it would just be on the ground. LOL I run Rotella 10w40 in them..
I have not got around to intruducing amsoil to my 08 Cummins. Plan is to run a bypass system due to the extra sooting of the oil with all the emmission systems on the truck..