I've got a 2002 3.0L v6 which has an occasional intermittent misfire which is only noticed at a stop light when idling. It doesn't have a routine pattern, it strikes randomly whether the engine is hot or cold. Once in a while you feel a jolt go through the passenger compartment. Majority of the time it never misses and runs fine. #6 cylinder has 180 psi while the other 5 cylinders have 220 psi. When oil is squirted into the cylinder the compression test goes from 180 to 220 psi meaning the rings are bad. Have replaced everything you could think of: spark plugs, wires, ignition coil, crankcase position sensor, camshaft position sensor, IAC. Even used the Pulstar high energy capacitor type spark plugs. So where do you draw the line? How low a compression is required to ignite the mixture? Am I missing something here? The mechanics at the dealership couldn't find anything else wrong except for the #6 cylinder low compression.

 

A rule of thumb that I always went by was that there should be no more than a 15% difference between high and low. According to that, your 180 cylinder is a tad bit more than 15%. I had an old twin cylinder John Deere that only had 50 psi but both cylinders were the same and it started and ran like a top. If it was mine, I would run it as is, at least until it gets worse.

 

+1 above
not necessarily bad rings. A badly scored cylinder gives the same indications but they can be verified by a bore scope exam.

The problem with a low cylinder is that you get crankshaft twisting which MAY lead to somewhat premature crankshaft failure or excessive wear, harmonic balancer failure, low life from the accessories. My thoughts are towards an injector issue or injector controller/wiring problem where the injector didn't fire. otherwise it would set a cyl misfire code. Kinda suprised it hasn't or you didn't mention it..

Having said all this, and not knowing your situation, your options are kinda blunt: fix it or live with it until something fails massively which could be another 50,000 miles. Most aircraft mechanics have a bore scope, harbor freight has em cheap. if it is scored, or you really think it is the rings, then you CAN just do the one cylinder even with the engine still in the vehicle.. Fun, NO, but doable.


my $.02 YMMV

 

You may have more than one problem.

Seeing as how you got positive results on the compression test after adding oil to the #6 cyl, & its compression rising to the value of the others, which were all nicely matched, it kinda suggests to me that #6 may have some varnish, gum, or carbon on the ring lands, causing the compression rings to stick. So maybe consider using an ester based crank case lube that's known to tidy up such deposits & see if compression on #6 will improve over time.

The random miss could be an ignition problem. I had such after the Dealer Tech cracked my #6 spark plug external insulator, when installing new heads & it would randomly act out, sometimes sitting at a stop light, sometimes on a pull & was especially bad in damp conditions, like rainy days. They found it with a wet down test. You could perform one at night with a spray bottle of water, by wetting down the coilpack & each plug wire, while looking for arcs & sparks, or an rpm change, while the engine idles. Yah I know you said the ignition parts are new, but that doesn't automatically make em good, I've seen plenty of bad new parts right out of the box, so make those new parts prove themselves!!!!
I'd also consider getting those aftermarket plugs out & install the specified Motorcraft or Autolite finewire iridium enhanced center electrode, with platinum pad sidewire design plugs & plug wires installed, as they're designed to take the double work load our "waste spark" ignition system puts on them.

The other thought that's come to mind with it being a random miss, is CCDF = Combustion Chamber Deposit Flaking. This condition can cause a random miss when combustion chamber deposits flake off & foul a spark plug, or get lodged under a valve at the seat, holding it open, such that it doesn't close completely. So, you might consider trying a 20oz dose of Chevron Techron Concentrate Plus, in the tank at the pump before filling, so you get good mixing & tank up with Chevron, Texaco, Caltex, or now Shell, which already have some PEA in their fuel ad pack. This will raise the treat rate 10X above pump gas alone & along with some daily runs with rpm at or above 3500 for 2-3 miles during the treated tank & running most of the treated tank out, will usually tidy up the valves, piston crowns, combustion chambers & fuel injectors, in one teatment. Regular use of fuels with PEA in their ad pack along with some high rpm blow it out runs, will likely keep things tidy in there.
If your going to try the ester oil in the crank case, do the Techron treatment first, so any deposits removed that get in the oil, come out with the old oil & filter.

EDIT: I forgot, since you say you've had the misfire code for 2 years, I'd look to a damaged cat converter too, as its had to process all of the misfire fuel, oil, ect, so do you have a cat converter low efficiency trouble code???? Maybe consider having the before cat exhaust back pressure measured.

More thoughts for consideration, keep us updated on your trouble shoot.