The additive in the oil starts to break down as soon as it heats up to high temperatures.  The engine in your vehicle will reach over 200 degrees almost every time you drive it.  History has proven that the 3,000 mile mark is a good interval to have your engine oil replaced.  You never want to just drain your engine oil out and put new oil in without changing the oil filter.  The oil filter will hold about a quart of oil.  If you do not change the oil filter when changing the engine oil in your vehicle you are combining your clean engine oil with deteriorated engine oil and this will lessen the effectiveness of the new engine oil you just put in your vehicle.