Nitrogen.
I agree on have a new valve put in too. gonna look into that on mine.
Here is what I can tell you with my experience.
2011, supercrew, I hit the trails a few times a year and rallycross the truck at least once a month.
Mine started going out near 50k. Before obvious signs of leaking, there was an increase in road input to the steering wheel at higher speeds, a subtle side to side vibration in the wheel
next, going through turns, cloverleaf express way type and other the truck would get a lot of feedback into the steering wheel and the truck felt like it was going to push through the turn.
During the last race, the front end was all sorts of unhappy. terrible feedback in the bumps and same issue in turns as stated above, only significantly worse. After 10 laps, 5 ish miles, both fronts had fluid leaking all over.
Now the truck is completely terrible to drive (fixes will be on the way)
If you notice some of these more subtle symptoms, I would serious look into replacement or rebuild before it becomes nearly undrivable, like mine is.