The service life of spark plugs in modern engines is a lot longer than in the past.

The service schedule for many vehicles these days won't include a fresh set of plugs until 100,000km or more. Changes to fuels and improved plug materials have had a significant influence on plug life.

What hasn't changed is that it's still vital that the correct plugs are installed to ensure proper vehicle performance, maximum plug life and avoid severe engine damage.

Fitting requirements 

Plugs have to fit correctly, so that means correct reach (the length of the threaded nose), correct seat design (the sealing taper or gasket), correct nose thread and diameter, plus the right electrode design and projection.

Running efficiency 

Plugs are designed to run at a temperature that ensures they don't foul up with combustion deposits, but cool enough to avoid pre-ignition, plug or engine damage. So it's critically important to use the specified heat range plug.

Some vehicle makers specify a resistor style plug. A ceramic resistor incorporated into the plug helps suppress high frequency energy that can be emitted and interfere with radios and other sensitive electronics. Such interference can even disrupt the engine's own operating systems causing driveability and performance problems that may prove hard to diagnose.

Replacing spark plugs

Plugs incorporating various precious metals such as platinum and iridium are often specified and this may increase their price substantially. However due to their different firing qualities, cheaper standard plugs should not be used as a substitute.

A modified engine, including those converted to LPG, may require a different heat range spark plug from normal specification and proper professional advice should be sought.

For standard engines spark plugs that meet the original manufacturer's specifications should always be used.