If fuel economy is your aim, I wouldn't run a high-mileage motor oil in your engine as they are formulated to be a tad thicker than other oils in the same grade. They lack the API "starburst" energy-saving symbol on the bottle for a reason.
ttoyoda, I'm not sure they all were? The 4.6L was originally spec'd to use 10W-30? That may be ... but my Dad has had a few of these and all I can remember was buying 5W-30 for his ... until the switch to 5W-20.
It could be that 10W-30 was the "in" oil at the time the engine hit the market. Most engines were spec'd for 10W-40 several years before 10W-30 and 5W-30 is still the most common weight. I'm not sure the auto companies agonized over these recommendations. The oil worked well enough ... and the EPA was pleased to see thinner and thinner oils being used to improve economy among the non-thinking mob of North American motorists.
(Most of us that read UOAs have seen 10W-30 shear down almost as fast as 5W-30 nowadays)
When 5W-20s first hit the market, they were LOADED with extra barrier anti-wear additives like boron and moly. Many still are. So, having the best additive package may have contributed to excellent UOAs ... despite the fact the engines were designed during the 5/10W-30 era.
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--- Bror Jace
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