Code P0308

Misfires can be all of the above plus. The way to figure out which part is the issue without spending a bunch of money would be to swap the plug and coil to two other cylinders. Unless you discover the plug is bad or if it has been a long time since the plugs were changed just change them all and swap the coil to a different cylinder. If the misfire goes away it was the plug. If the misfire moves to the cylinder you put the coil on, the coil is failing. If the misfire stays on cylinder eight then it could be a dirty/failing injector or a problem inside the cylinder.
With OBD codes they only point you at whet the computer sees as a problem. For this code the only thing the computer actually detects is that number eight cylinder is not contributing with every power stroke as it should. It does not know why.

Yes, there can if the coil actually fails and is causing a constant miss or if the PCM sees an issue inside the coil itself causing a problem. If it is an intermittent misfire the coil code will not set because the PCM still sees the coil firing. That is why I suggest just moving it to another cylinder for testing. I did not say to replace them all. It may be carbon tracking inside or have a simple connection issue. So if you moved it to cylinder five and now the miss is on cylinder five you suspect the coil. If the miss stays on eight then it is the plug or internal issues like a valve or compression loss or injector that is dirty or sticking.

If the plugs have not been changed in a while they will all have worn gaps. Those worn gaps stress the coils and can set misfire codes as well. So if you remove the number eight plug and see it is worn you know the other plugs are likely just as bad, so you replace them as well.

You can do a relative compression test using an oscilloscope to see if the compression is close to the same in every cylinder. As most people do not have one I do not consider it a DIY test. You could do a physical compression test. Not hard to do but it means pulling every plug, which if they are worn would be a waste of time to reinstall.

What test equipment do you have?