Crash course in hydraulic valve lifters.
WAAAAY back, all engine had mechanical valve lifters, or "tappets" (which is why the guys on Car Talk call themselves the Tappet Brothers). They had to be adjusted periodically, when the engine was hot. Not rocket science, but a messy, nuisancy job that lots of owners never had done.
Along about the mid-1950s the manufacturers came out with hydraulic lifters. The deal with them is that they are self-adjusting. Inside each one is a spring and a check valve, beneath a plunger which is what actually contacts the bottom end of the pushrod. When you fire up the engine, oil pressure is supposed to push the plunger up and maintain it in contact with the pushrod, while providing enough pressure to lift the valve by the specified distance.
Hydraulic lifters fail in two modes: either the check valve goes bad and they bleed down too fast, or the oil feed orifice gets clogged and they don't get enough (or any) oil. If the latter happens, the plunger doesn'tride high enough to keep the pushrod in contact with the rocker arm when the lifter is at the low point of its travel, and the "tick" you hear is when the pushrod starts to move up and makes contact.
If it's happening all the time, eventually it can cause wear on the cam and the rocker arms. If it only happens when the engine is cold, or only at idle, it's probably not worth replacing the lifters. The reason that job is so expensive at a dealer is simply that on the 4.0L you have to remove the head to replace the lifters.
Heavier oil will address the problem if it's a case of the lifters bleeding down too quickly, but will not address the problem of dirt. You need a cleaner to have a prayer of fixing it that way. I like Marvel Mystery Oil for that purpose. The directions are on the can. If you have noisy lifters, you want the "purge and clean" dose, not the normal "I do this at every oil change" dose. It's not guaranteed to work, but it often does. Sometimes helps to combine the Marvel purge with a bit of Mexican tune-up -- run it up to high RPM for several seconds. I prefer to do this in 2nd or 3rd gear on the road as opposed to in neutral in the driveway,but that's personal preference. The idea is to maximize the oil pressure so any specs of dirt can be pushed through and out of the lifter passges.