Engine oil change

Engine oil has limited life - after a certain point it starts losing lubricating qualities and carbonizes. Once it happens, the engine gets contaminated with carbon deposits or sludge (see the pic.) that significantly shorten engine's life. When you change oil at or before manufacturer suggested interval, you change the oil before this "carbonizing" point, engine remains clean and once refilled with new oil ready to work hard again. If the engine oil has not been changed for long, carbon deposits start clogging the oil pick-up screen decreasing oil supply and increasing friction. Through the engine ventilation system the same carbon deposits build up inside the throttle body and EGR system causing rough idle and possible check engine light. Compression decreases and engine start wearing much faster. If you don't remember when you changed the oil in your car last time - just check the oil on the dipstick. And every time you change the oil, the oil filter should be replaced as well.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 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
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.
The best schedule to change engine oil is accordingly to owners manual instruction or mechanics advise.