I have both a 99 (5.4) and an 01 (4.6) F150 both are around 180k miles. I am about to do oil changes to both and I'm wondering if I should switch to a "high mileage" oil it if standard oil is still ok? Also I live in SE Texas but have so far only used the factory recommended 5W20 oil, should I go to a heavier oil due to the extreme outdoor temperatures?