As many have said here we use 10w-30, I cannot explain why the owners manual is worded as it is, but it is. My 6.7 is my work truck, its loaded every day, whether it be towing or having 4,000 lbs.+ in the bed, or both. I have let the UOA data confirm that 10w-30 is fine with what I'm doing.
I think the 5w-40 and not a 15w-40 recommendation is purely for fuel economy, I know these trucks aren't CAFE mandated but cold engine operation robs the most in fuel consumption.
I think the 5w-40 and not a 15w-40 recommendation is purely for fuel economy, I know these trucks aren't CAFE mandated but cold engine operation robs the most in fuel consumption.