The manual says that 10w30 is preferred and to use 5w30 if you experience sub zero temps. I'm assuming because the oil clings to the parts a little better. My instincts tell me to take it down to 5w so I can get better flow at startup but I can't help but think the engineers chose 10w for a specific reason. I know lots of people run 5w40 and 15w40 in this engine as well so if anyone could give me some insight that would be great. I've already decided on rotella with a pureone filter for my next oil change all I have to do now is settle on a viscosity.