I ran 15w40 all last winter and i forgot to plug my truck in 1 time and she didn't start. It barely turned over, but as soon as i got home and plugged it in for about 1-2 hrs she popped right off. I now run 5/40 syn. I know you said you don't want to run that, so i would go with 10-30 then. My bro runs 10-30 in his 6.0 and never has any problems. The syn is good oil though. It makes the engine run quieter and its supposed to help with mpg. IDK JMO, but i like it.