I think a good HM oil such as Valvoline MaxLife Blend 5w30 would be a good choice. Its reasonable in price (Walmart 5 qt.jug is about $14.50).
Do an oil change at about 3000 miles to remove alot of sludge that has built up..After that, 5000 mile oil changes should be OK
Put on a good filter like a Purolator Pureolator PureOne. WalMart does not carry them.
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2003 Ford Focus SE 2.3 L / 82K
Valvoline MaxLife Syn 5w30 / OCI 7000+/-
Purolator PureOne PL 10241