nothing wrong with running a 5w-40 oil, which is what Id use if I wanted to go a bit heavier.
Ambient temperatures may effect cooling capacity a bit, but unless youre driving like you stole it, the oil temps are likely still OK. That said, if the manual suggests a 40wt for your conditions, then why not?
Rotella syn 5w-40 or schaeffer's 5w-40 would be my two picks for quality oils at reasonable prices. I still would venture toward the thinner at startup end, a 15w-, while likely a non-issue, still can be slower to pump up than a 5w-. Not anissue on big rigs where the engines are onnearly constantly, but I prefer a bit more surity that Ill have quick pumping at any conditions.
Sut all of that said, I cant find fault with you wanting to run delo 15w40 either... I would suggest you do some UOAs to see what you have going on...
JMH