I change my oil reguarly but before it's time to change it or very close to it my dipstick will have no oil on it if I make a turn or on uneven ground my car will say oil pressure low.
If you have the 4.6 liter V8, then they tend to develop lots of oil leaks, which would explain why your low oil light comes on. Take your car to a shop that offers free 'top offs' many do, that way once a month or so you can get your oil topped off, until you decide to fix the reasons for the oil going away
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