thats what happens when you get your car serviced at garages, they usually only change plugs and oil if they think you're not very car wise, so the leads, air filter, fuel filter, pollen filter, coolant, brake fluid, etc etc etc rarely get changed. i've seen cars with full service history and they've got less than 5% antifreeze in the radiator, cracked silicone on the HT leads, a birds nest worth of junk in the airbox. its impossible for a regularly serviced car to have any of those things. dealer services are usually better but they cost the earth, DIY is the only way to be sure its been done properly