I had a 97 F150 with the 4.6 modular V8...You were able to use 10w-30 in that engine...
I was under the impression that the basics of their modular engines havent really changed since then.

My questions are.....Did the 5w-20 requirement come about just to improve gas mileage and for starting in colder climates ???? Did the tolerances in the later engines change to warrant this ???

Would using 5w-30 hurt anything or jeopardize the warrantee in my 09 5.4 V8 ???, especially if you live in the warm south like me. 5w-20 is soooo thin. I would give up a fraction of a mile to the gallon for the better protection of a slightly thicker oil...

Any ideas ????

 

The tolerances are tighter in the newer engines is the reason behind 5w-20. The Lincoln dealer told me that on my wife's 02 LS with the 3.9L V8 and told me under no circumstance should I put 5w-30 or 10w-30 in the engine.

I knew someone that used 10w-30 instead of 5w-20 in a 2005 F-150 and the engine went out and ford refused to warrant the claim over the use of 10w-30.

 

5w20 is just fine for these engines and works quite well. In fact Ford now recommends that oil, even for your old vehicle. Thicker does not mean better.