Also from Tx. My 5W-40 synthetic (have used both Valvoline Premium Blue Extreme and Shaeffer 9000) performs just fine in the heat. Oil analysis data shows that it is fine. All oils will shear in our 6.0L engines - some more than others ..... but it is not a function of being synthetic 5W40 or not. As 69cj stated, the 5W40 performs as well as the 15W40 at the high temperatures. That is defined by its "40" rating. The wider range is achieved with superior base stocks as compared to conventional oil.

As I posted earlier - Ford is selling a 5W40 oil now and recommends it for ambient temperatures over 100 degrees. They also have kept the same recommended oil change interval - 7500 miles normal (or 6 months) and 5000 severe service (or 200 engine hours).

Also fyi - it is the fuel that lubricates and cushions the injectors. The oil is a hydraulic fluid

 

Too each his own and whatever your truck seems to prefer. Based on the threads here and on other sites you are in the minority. But do what is best for you. No debate here.

 

Oil shears in all internal combustion engines, even my Harley shears oil molecules. Some engines shear more then others.