Ford recommended the switch from 5W-30 to 5W-20 with the 2001 model year. Prior to that 5W-30 was recommended for all 4.6 and 5.4 engines. The change was made in 2001, as I understand it, to incrementally increase the mileage due to reduced friction. Nothing had changed in the motors to dictate the change to the 'lighter' oil. Check out the Lightning forum as you could read all day on this subject. I've run Mobil 1 5W-30 in my 2001 L from 600 miles and have never had a problem with it.