How often should you change the oil in your car? That depends on what kind of oil you’re using. Synthetic and conventional have different life spans, but one thing is sure. You should never try to stretch the span of your oil in order to save a dollar. You could be causing real damage to your engine and you may, by not taking the car into the mechanic in a timely way, be missing out on seeing a real problem that could cost you plenty down the road. For general rule of thumb.. for conventional oil, change the oil every 5000 miles at the most, while synthetic will let you take it up to 10K before you need to be sure that you get it changed.