I doubt it's the resistance spec. Resistance is usually 5k ohm. 5k ohm vs 6k ohm makes no difference. Resistance is only to prevent the spark plug from acting like a radio-frequency transmitter antenna.
Is this an NGK plugs? 4, 5, 6, etc. would be the heat-conductivity spec, which makes a huge difference. Using the wrong heat conductivity will cause preignition (too little heat conductivity, spark plug running too hot) or spark-plug fouling (too much heat conductivity, spark plug running too cold).