New ignition systems are very precise and some use wasted spark DIS which has a reverse current flow on half the plugs. If two plugs share a coil, in a distributor-less ignition system, the coil is usually wired in series in between the two plugs, pulling up current from ground from one plug (flowing current in reverse) and pushing down current to ground from the other plug (flowing current in the conventional way) and ground completes the circuit between the plugs.
Plugs from different manufacturer will vary and they have several different models of plugs with different features. Like a single platinum plug designed only to work with current moving in the intended direction, so that the plug lasts much longer, but use it with reverse current and it will erode away much faster than a standard plug. That is why there is also double platinum (platinum on both sides of the arc for these ignitions).
So people use the wrong plug, for one of many reasons, despite it being one of the many different "standard sizes" that should fit your engine, one of its features or how it varies from other manufacturer's resistance, quench area, etc, make for poor performance. So some plugs work well in certain engines and others work poorly.
We say use the Original Plug, by manufacturer and part number, the engine was designed to use. I "suspect" more than one brand/model plug will work fine, but there are more than a few that won't work well at all, to avoid a problem, its recommended to only go with what has proven to work, and we all know the original plug by manufacturer and model has proven to work since it comes with it off the assembly line.
As mentioned, Gap affects the resistance of the overall circuit and gap, shape of the electrodes, open/closed area around the spark, also effects the effective ignition of the air/fuel, which can be very complicated. I'd also wonder when a manufacturer recommends a different gap than the engine manufacturer, is it done from their own personal testing? OR are they just recommending a generic gap regardless of the actual engine. Keep in mind, the whole ignition creates a circuit, spark plug cables often degrade and their resistance goes up, depending on the type of the cable, resistive or suppressive (inductive), they eventually need to be replaced with new to get back to the original overall resistance.
Finally, over the years, Chrysler has switched back and forth from a coil for every plug to one coil for two plugs, the wasted spark with reverse current flow. Commanders have had both systems over the years. So what might work for one Commander owner, may NOT work at all for another Commander owner because their ignition systems are different.
Personally, I just stick with the OEM brand/model plugs and use the engine manufacturer gap recommendations, and avoid the ambiguity. That's NOT to say some other brands/models of plugs may work just as well, but you may pick a plug that doesn't work as well.
Last edited by Mongo; 03-21-2016 at 02:55 PM.