I pulled this off one of the 2.8 forums. It's more or less the same description as to why I didn't run a 5w30 in my 5.0. Newer engines are designed much differently than 5-6 years ago.
"5W-40 synthetic is an excellent oil but, one should never use a higher viscosity than recommended in todays
engine. There are a number of reasons, one being VVT is dependent on proper pressure for proper operation. Another is the precision of oil delivery to cam lobes and followers, bearing races, etc. The higher viscosity does not flow the same as the recommended (lower) viscosity. You may not notice a difference, but over time, your
engine wear does.
This is not to say if your rating permits you should not use the highest viscosity permitted for specific conditions."
Now the 0w40 is fine to run and there is actually a debate if it's not really the better oil. The flash point between the 0w40 and 5w30 is within 20-30 degrees Celsius IIRC.