I read the factory recommends 15w-40 but many on the forums recommend the 5w-40 instead. I read the thinner oil is better for stiction and cold weather start up but is it adequate protection when it gets warmer?

 

I use 15W-40 in the spring/summer/fall and switch to 5W-40 in the winter. Where I live it gets below 0 F just about every December, sometimes as low as -20 F.
This rotation works well with the number of miles I put on the truck. I run 5W-40 for 5,000 miles, which takes me into late April, then 15W-40 for 5,000, followed by 15W-40 again for another 5,000. If I don't get to 5,000 by the time December roles around I change it a little early.
This rotation is a lot cheaper than running 5W-40 all year long, which is what I used to do.
When the truck was new, until it had 60,000 miles I ran 5W-40 all year round on a 7500 mile change interval. I started having stiction towards the end of the change period, even with 5W-40, so I pulled this back to a 5,000 mile interval. I've heard people say that the 6 liter diesel shears the oil, making it thinner, and this reduces it's protection capability, which I'm sure is true, but I personally think the black soot that collects up in the oil is what eventually causes the injector stiction issue.
The truck runs great all year round now; starting is easier in the winter and I think 15W-40 makes it run better in the heat.

Don't know why you said the factory recommends 15W-40. The manual lists everything from 0W-40, 5W-40, 10W-40 and 15W40, depending on the temperature.
I guess maybe your statement is based on your local temperatures?

 

I run Amsoil 5w40 with a by-pass filter. These motors love Syn oil in a 5w40 and the max is 5000mi even with Syn and a by-pass because of the shear of the oil by the injectors. The stock Ford filter, filters down to 20 micron and the by-pass filters down to 2 microns.
2 stroker