Regular oil changes are very important to keep your engine in top condition.
You must also chose the right oil for your engine. Older engine's use thicker oils while more modern performance engine's use thinner engine oils. My preference is a synthetic oil as they do a better job and reduce sludge build up in your engine.
It also depends on how hard you run your engine. I like to hammer mine so I change my oil regularly about every 10,000klm.
You can also save yourself some money doing it yourself at home.
Happy motoring from HM