so i did a search and found this page http://blog.bluespringsfordparts.com/411/synthetic-oil-f150/ i guess even the first 5.4 was said to use 5w20... truck has just about 100K on it. should i go from 5W30 to 520? i know ford spends countless $ designing engines but im a little hesitant to run 5w20 in a truck that is used for daily driving and towing boats/trailers on a semi-regular basis =/ what do you think ? and FWIW. i have ~ 4k on the full synthetic 5w30 and ive added about 3.5 quarts to keep it topped off. i cant see 5.20 making that better.