It depends on the climate between now & the next oil change. 5W is what modern vehicles are built with because it reduces internal friction & raises CAFE. But it's not necessarily the best for your particular vehicle in your particular region/season. And as the engine ages (wears), you should go up 1 level for every 1.5-2Kmi. I run 20W50 in mine right now because it's close to 400Kmi, but I've taken good care of it.
The 5.8 has nowhere near the tight clearances as the Triton, so you can probably use just about anything in it. However, the factory spec was 5W30, and that should be stamped on your oil cap.
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2003 F150XL 4.2 short cab/bed 4x2 5 speed manual 3.55 LS
The specs call for 5w-30 and is backward compatible to 5w-20 but I wouldn't recommend it. Unless you have issues with it using oil, the 5w-30 is what I would run in it especially in the Texas heat. Thinner oils collect and release heat faster than the heavier viscosity oils. If it's using a little oil, move up to a 5w-40. The 5w means it will get into circulation a lot faster than a 10w or a straight weight oil. It only comes into play on cold start up. The wider the spread, the more chance there is that the formulator has used a polymer of some sort to make it. So a 5w-30 may not use any Viscosity Improvers but the same line of oil in a 5w-40 may use a lot. Polymers are known to sludge over time. My 91 Bronco with 130,000 miles on it uses maybe a half a qt in 5000 mile changes. I'm using a 5w-30. I've ran the heavier oils only to watch the fuel mileage go down and the heat go up per laser probe. Coolant temps stay the same but the metal is running hotter. Choose wisely grasshopper.
I'd have to agree with Labnerd. Unless your bearings are shot and you can't maintain oil pressure, there is no reason to use a heavier oil than what the factory specified. I've read one theory that engines use less oil through the rings with thinner oil because the oil rings can more easily scrape it off. Heavier oil would help to reduce consumption through worn valve guides. I would use a Heavy Duty Engine Oil (HDEO) rather than a Starburst oil in an older vehicle. Multigrade 30-weight dino HDEOs are usually found as 10W-30 and synthetic 30-weight oils are 5W-30 and 0W-30 because they flow well in cold temperatures. And no, a 0W-30 synthetic HDEO will not be too thin, even in Death Valley.
If you're looking for an owner's manual, you can download them from the Ford Owner's Manual Downloads site but it only goes back to 1996.
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