Check the engine oil regularly, I'd recommend at least once a month or even more often if the car has high mileage. If you aren't sure how to check oil level, follow the link: Illustrated Maintenance Checklist   Driving with extremely low oil level (less than min.) or with low oil pressure warning lamp on may cause serious engine damage   If you note, that oil pressure gauge indicates extremely low oil pressure - have your engine inspected as soon as possible. - While checking the oil level, look at its condition. Check the image at the left. If the oil is black like on the right image, I'd suggest to change it. - Always use only appropriate engine oil type (usually you can find it on oil cap or in the owners manual). - Check your parking space for leaks. If you find any, fix it before it results in more serious vehicle problem

Synthetic oil vs. conventional mineral oil

The advantage of synthetic oil is that it can withstand higher temperature and can work longer without losing its lubricating qualities. It doesn't get thicker at below-zero temperatures providing good engine lubrication at a cold start. However, since it's more "thinner" a high-mileage engine filled with synthetic oil will more likely to develop leaks and you will more likely to hear lifters tapping noise at a start. Therefore, If you have low mileage or turbo engine and driving under heavy conditions such as high temperature, excessive load, long intervals without an oil change, etc., or simply want to provide extra protection for your engine, synthetic oil may be a good solution. But I don't think it's worth to use synthetic oil in high-mileage engines - thicker mineral oil will provide better protection as long as you change it regularly.