Ford 6.7 - what oil do you use?

I know the question will elicit various responses, but a post regarding a 2012 Ford 6.7 engine failure on another forum prompted the question. He used 10w30 non-synthetic oil which he stated was changed every 5000 miles. Traditional wisdom says this would be a good choice. My manual (2013) clearly states using 5w40 synthetic for severe duty service, which is described as extensive idling, lots if low speed driving, and towing heavy loads. This describes my driving as we travel extensively towing in the Western states. This is my second 6.7 and I've always used the 5w40, but also find it very strange that dealers (I always use them for the change) don't know what the manual says. I'm always questioned about the choice and show them the manual. I've gotten in the habit of calling ahead as many times they have to get the oil for my change. The manual also lists other choices, but specifically the 5w40 synthetic for severe duty. Why doesn't everyone do that for towing heavy 5th wheel trailers?