If you had a misfire code for cylinder 8 (P0308) then you probably do have a misfire in cylinder 8. Most scanners also have a misfire counter that you could see all eight cylinders and see if there are also other cylinders that may be misfiring but not enough to set a code yet.
Misfire could be a spark plug. could be an ignition coil. could be spark plug
wire. could be an injector. could be something mechanical.
spark plugs would be an easy diagnostic procedure to see if that takes care if the miss. Scoping the ignition pattern and current ramping the coil and injector would give you more information but time wise, replacing the one spark plug
and looking at the misfire counter to see if the miss goes away would be cost effective. If it does take care of the miss I would recommend that you replace the rest of the plugs as maintenance. (how many miles are on these plugs in your engine?)
Letting the misfire continue could cause damage to the catalytic converter. (too rich) could burn a valve or damage a piston (too lean, detonation)