6.0 oil questions

[Diesel625]

Got a question. I run a kdd stage 1 long block in my 06. I’ve ran nothing but schaeffers 15w40 since it’s been broken in. I’ve been wondering if anyone runs a 5w40 or 10w30 in this particular engine? Kdd recommends schaeffers 15w40, but is there a particular reason for it? I know several people that swear by the 10w30 or 5w40. I tow heavy and summer weather is consistently over 100f. Winter isn’t bad. We occasionally get down in the teens. No issues starting or stiction in my case.