My Porsche handbook recommends the spark plugs in my 2001 3.2l engine to be replaced every 80,000 km (48,000 miles) or every 4 years.
I've driven only 30,000 km (19,000 miles) since I replaced the existing plugs, but now the magic 4 years is up.....
What's the consensus about changing the plugs every 4 years - valid or not ???