Why change your vehicle's oil?
As the oil lubricates the internal moving parts of your engine not only does its' lubricating properties break down but it also picks up grit and other particles. It is very important to the longevity of your vehicle to change your oil on a regular basis. Somewhere between 4k - 10k is sufficient for most vehicles depending on your driving conditions and habits.
Why does the oil break down?
Because of the friction, heat, air, and particles the oil begins to oxidize and loses its effectiveness. It begins to develop sludge, thickens and deposits begin to form, which can impede the free flow of oil.
What is the difference between the various types of oil?
This is one of those questions that one wonders if the salesman is being honest or does he want you to buy the nore expensive item. The bottom line is synthetic oil is a better quality oil but may or may not add miles to the life of your car. What I look at are all the costs associated with the purchase, upkeep and driving of my vehicle and divide that by the total miles driven. This gives me the "cost per mile" of my vehicle. I try to get the lowest cost per mile without compromising the health of my vehicle.
I know it is hard to get up front honest advice when talking to professionals that want your business now. We on the other hand want your business now but would like to have it a decade from now. We do not want to oversell you, nor do we want to let you down by not warning you of the pitfalls of the road. Our advice when it comes to oil is to increase the quality of your oil as you add miles to your vehicle. Ultimately you have to decide what is best for you.
Check out this article by Consumer Reports