The Ford dealer has told me *not* to use 10w30 oil or 5w30 oil in my 03 5.4. They said that the 5.4 since 01 has called for 5w20. The manual and the oil cap are all in agreement about the 5w20.

Aside from the fact that mobil1 5w20 is impossible to find (they stopped making the 0w20 apparently too), does anyone have knowledge about why its so important to stick to the "20" side of the equation on these trucks? I understand that during the winter the 0 or 5 is better than the 10 to make sure oil is flowing at ignition when its zero degrees out (Chicago). But everyone always looks at me funny when I tell them my truck uses 20 weight oil for the summer. And it does sound strange - its not some dinky car, its a 5000 pound truck. If I was towing, would 20 weight *really* provide the lubrication to protect the engine? is 30 or 40 weight going to harm the truck in some way like the 10 could conceivably during winter?