I've got the HT15's in our Expedition and after 50K miles, it's started to occasionally throw a temporary misfire warning if you start to really get on the throttle. I don't think I'll be leaving them in for a 100K.
As for what to watch out for with the 5.4l, if it starts having what sounds like lifter noise (different from the cam phaser ticking) get the follower rocker arms checked. Bad stuff will happen in the valve train if you let those pound and clatter too long. Another common problem is the seals on the timing chain tensioners can blow out and starve the top end for oil. If you have timing chain slap noises, you should have it addressed. Also, its wise to have the hot idle oil pressure checked on higher mileage engines if you suspect an issue. Don't believe the oil pressure gauge on the dash as it's essentially a dummy light that will always read the same as long as the engine has over 7 psi of oil pressure. I think it's a good idea to avoid extended idling with these engines, too, to save wear and tear on the valve train.
Lastly, the oil life monitor on those is set to a static 5K mile interval up to 07, and I would not run it longer than that. I use 5w30 in mine, BTW.
As for what to watch out for with the 5.4l, if it starts having what sounds like lifter noise (different from the cam phaser ticking) get the follower rocker arms checked. Bad stuff will happen in the valve train if you let those pound and clatter too long. Another common problem is the seals on the timing chain tensioners can blow out and starve the top end for oil. If you have timing chain slap noises, you should have it addressed. Also, its wise to have the hot idle oil pressure checked on higher mileage engines if you suspect an issue. Don't believe the oil pressure gauge on the dash as it's essentially a dummy light that will always read the same as long as the engine has over 7 psi of oil pressure. I think it's a good idea to avoid extended idling with these engines, too, to save wear and tear on the valve train.
Lastly, the oil life monitor on those is set to a static 5K mile interval up to 07, and I would not run it longer than that. I use 5w30 in mine, BTW.