I was told the same, use 5w 20 or void your warranty. The problem is that nobody but Ford makes 5w20 oil. What happens if you are not near a Ford store?
Use some 20w-50 motor oil that will keep the oil from bypassing the valve guide seals so that your 4.6 V-8 will not burn any oil. Because it is a common problem for 4.6 V-8's to burn oil when it goes by the valve guide seals.
Nobody else makes 5/20 oil? My husband just picked up his new company truck with 5.4 engine - I noticed that it needed 5/20 oil. Went to Pepboy's and picked up a few quarts of 5/20 Pennzoil, no more cost than normal, just in case we need to top it off. By the way, this truck with big engine, 4wd and some extra weight is getting nearly 20 mpg - some of that may have to do with the lighter oil. My 1999 with similar equipment breaks 17 mpg, but that's max with 5/30 oil.
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