I had a 2006 F150 XLT (5.4) before I traded it off for the '15 EL. There was around 150-160k miles on it when it moved on the greener pastures, IIRC. I had one problem with it in stock form and it was because when I bought it, the plugs had been changed but not tightened which caused a gas smell and ticking sound, obviously, as compression and fuel were bypassing the plugs. A bit of work torquing them to spec and I never had to mess with the engine again. I change my oil at 5k miles with regular 10W-30 dino oil and, sometimes, run some Seafoam through the engine before the oil change to clean things out.
I didn't have any problems with that truck until I decided I wanted to put 35's on it and trusted the wrong mechanic to do the work. It's a long, expensive story so I wont get into it but even still, I never had any issues with the truck as far as the engine and transmission. After I started modding, things went to hell but it was not the trucks fault. It drained my wallet and ruined an otherwise fine truck in the process.
I used to be on the F150 forums and there were complaints of phasers and plug breakage but if one went back and researched the people who were having issues, I generally found there was some modifications done in the past for more power or to make it faster, or, even no issue at all. Some just read posts and were convinced their truck would fail, so they tore into the engine to make the 'mods' and tore it up in the process. One time this guy was complaining of ticking and fuel smell to which all the respondents had him convinced he needed to tear his engine apart and 'lock his phasers'. He was stressed over the expense until I told him my plug story which fixed his issue. I say all that to say this; be wary of what you read online. Generally the only people who post are those who are having issues and those issues could be self-induced.
We also had a 2007 EL with the 5.4 and loved it, up until it got over 100k miles then it seemed we started having some electrical issues, not in the engine or anything but in the AC unit and other internal nuisances. Well, I take that back. It did start missing once but a plug and COP change resolved the issue. Anyway, after about $3k in repairs, I finally got mad and traded it off when in reality, all the problems were fixed and the next owner got a fine vehicle.
I do that. I repair vehicles for other people before trading them in. I also buy stocks, gold and silver when they're high and sell when they're low.
I say all that to get to this point: if it's been taken care of and left alone, you will probably have a fine vehicle that will give you years of enjoyment. There will be issues arise, as it is with all vehicles, but that's the gamble one takes when buying used. Just give it a good look-over, drive it hard before buying and, if possible, let it sit and idle for 20 minutes before killing it. If it all checks out and you can financially swing it, take it home and enjoy it.
