The other reason they changed oil specs on the 4.7 (and later, all engines) was due to issues with sludge buildup on the 4.7's oiling system.

 

This and saying the 4.7 is in part a Mercedes design, are some pervasive myths. (It's actually an AMC design.)

Every manuf has moved to thinner, full syn oils. The 4.7 is no sludge monster. However, it is more susceptible to the ravages of poor maintenance than the LA motors. I mean the most basic of things like not changing, not checking and keeping the level up while running on crappy conventional. It is a modern motor with tighter tolerances.. It's got aluminum heads on an iron block, so you also don't want to overheat it. Yes, you will sometimes see milky buildup in the oil cap, cause the filler neck stands tall off the motor, causing the oil mist to cool and harden with the lower temps.

Personally, I won't run conventional in anything... not even my lawn mower. Not when I can get the big bottle full syn at Wally world for $21. They hold viscosity longer, protect better, keep things cleaner and flow faster when cold.