IRST, ALL OIL BREAKS DOWN. That generally will include basestocks and additives. Without focusing on performance characteristics, the most significant difference from one oil to another is how quickly breakdown occurs. Although there are many factors that contribute to the breakdown of an oil, heat is one of the most important. Depletion and decreased effectiveness of oil additives is also important, but that will be discussed later.

Petroleum oil begins to break-down almost immediately. A high quality synthetic, on the other hand, can last for many thousands of miles without any significant reduction in performance or protection characteristics. Synthetics designed from the right combination of basestocks and additives can last indefinitely with the right filtration system.

As alluded to above, the first major difference between petroleum and synthetic oil is heat tolerance. Flash point is the temperature at which an oil gives off vapors that can be ignited with a flame held over the oil. The lower the flash point the greater tendency for the oil to suffer vaporization loss at high temperatures and to burn off on hot cylinder walls and pistons. The flash point can be an indicator of the quality of the base stock used. The higher the flash point the better. 400 degrees F is the minimum to prevent possible high consumption.

Today’s engines are expected to put out more power from a smaller size and with less oil than engines of the past. Therefore, the engines run much hotter than they used to. That puts an increased burden on the oil.

Even the best petroleum oils will have flash points only as high as 375 and 420 degrees F. For today’s hot running engines, this may not be nearly enough protection. Just about any synthetic you come across will have a flash point over 430 degrees. High quality synthetics can have flash points over 450 degrees (Every AMSOIL synthetic oil has a flash point of over 450 degrees F with some exceeding 475 degrees). As a result, you will see little noticeable breakdown due to burn-off.

Just as important is the way in which petroleum and synthetic oils burn off. As a refined product, petroleum oils molecules are of varying sizes. Thus, as a petroleum oil heats up, the smaller molecules begin to burn off. Since the ash content in most petroleum oils is very high, deposits and sludge are left behind to coat the inside of your engine. In addition, as smaller particles burn off, the larger, heavier molecules are all that is left to protect the engine. Unfortunately, these larger particles do not flow nearly as well and tend to blanket the components of your engine which only exacerbates the heat problem.

Synthetic oils, because they are not purified, but rather designed specifically from the ground up for lubrication purposes, are comprised of molecules of uniform size and shape. Therefore, even if a synthetic oil does burn a little, the remaining oil has the same chemical characteristics that it had before the burn off. There are no smaller molecules to burn-off and no heavier molecules to leave behind. Moreover, many synthetics, have very low ash content. As a result, if oil burn-off does occur, there is little or no ash left behind to leave sludge and deposits on engine surfaces. Obviously, this leads to a cleaner burning, more fuel efficient engine.

As a side note (as it really has little bearing on when to change your oil), synthetics do a much better job of “cooling” engine components during operation. Because of their unique flow characteristics, engine components are likely to run 10 to 30 degrees cooler than with petroleum oils. This is important, because the hotter the components in your engine get, the more quickly they break down.