5W40 or 15W40 in 6.7 CTD?

There have been hundreds of thousands of Cummins owners that have gotten hundreds of thousands of miles out of their truck running regular 15w40 even in colder climates. Not saying that running 5W40 is bad or not needed, but most who do only do it for "cheap insurance" because they "feel" that it will extend the life of their engine. How long while it extend the life of the engine? They don't really know, but it makes them feel better.

Could be that 5w40 extends the life of the engine an extra 10k miles or 100k, but if you are only keeping the truck for 300k miles and 15w40 will easily get you there then what is the point of spending extra money on something that will extend it past that? You are just spending extra money that will only benefit the next owner.

Basically, most(not all) are doing it based on emotion rather than actual data or need. Some do actually need to run 5w40, but most do it because makes them feel better that they are doing a good thing extending the life of their engine. Not saying this is bad thing because a lot of people(including me) spend lots of money on things that make them feel better, like beer :B......

100% run 15-40. If it's synthetic the pour point is not much higher than the 5-40.

This is what the owner's manual says:

Follow Ford’s recommendations. My gas Chevy is 0W20. Light stuff!

15-40 should be just fine

I am always in favor of doing what the owner's manual, written by the people who designed and built your vehicle, says to do rather than people on an internet forum say to do.