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?

 

Keep using the same as you have been.
There is no reason to change.
Going heavey is old school thinking.
The motor has oil pressured cam chain tensioners.
Using heavey oil puts excessive tension on them during cold starts.
The motor is not a push rod type.
Motorcraft oil has finally gone up in cost. It 'was' the best deal out there for a blended oil.
If you use some other 5w20 non blend, at least get the oil with a little more zinc for a little extra wear protection.
Some of the oils have seal expander additives.
Avoid them if you don't need it.
At the mileage you have, notice the oil will color quicker due to ring blow-by. A couple thousand miles after that it's time to change it and the filter each time.
Just for compairison, here in the notheast climate, I have been changeing every 5000 + for the life of the motor using 5w20 and MC filter.
The truck has been punished towing as high as 12,000 gross combined weight in dead heat of summer.
Oil pressure with an external gauge is nearly 70 plus at cold start, 25 at hot idle and nearly 50 hot normal running speeds
The motor is quiet as new at 176,000 miles.
Good luck for whatever it's worth.

 

I agree with bluegrass.. I've always used Motorcraft 5w20 and MC oil filter in both our 03 F150 4.6 and our 06 escape 3.0 never ever had an issue. Cheapest place is Walmart $18.33 for 5qt jug $4.37 for one qt and $3.97 for filter