Just bought a 2001 f-150 with a 5.4 motor. the owners manual says to use
5w-20 oil in it. That sounds awful light to me, we have always used 10w-40 in our cars and light trucks. According to the sticker on the window, the previous owner used 10w-30.

So what weight should I use? Is there a reason to use 5w-20? I live in sw Missouri, so our temps vary from -10 to 110 degrees during the year.

 

Welcome to FTE!!

IMO, you should use the 5w20 if it is under 100K mi, and then bump up to a 5w30 or 10w30.

10w40 is a bit heavy IMO, and not as shear stable as a 5w20 or 5w30. The modular motors don't respond well to weights higher than Xw30 and tend to have higher upper valve train wear.

This topic has been covered my times in the Oil and Lubrication Forums, you'll find lots of information on this very topic.

 

I agree. These mod engines need oil to the top of the engine quick on start-ups and 5w-20 does well. I have towed 7000lbs with the a/c on in 98 degree weather and have had no oil related problems at all using Motorcraft 5w-20 with a Motorcraft filter.