First off, 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 almost 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 absolute 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 390 and 440 degrees F. Some actually have flashpoints as low as 350 degrees. For today's hot running engines, this is not nearly enough protection. Just about any synthetic you come across will have a flashpoint over 440 degrees. Premium synthetics can have flashpoints over 450 degrees with some even reaching as high as 500 degrees. That's a big difference.
As a result, I think that it's quite obvious that these high- tech oils offer a substantial benefit when it comes to potential breakdown due to burn-off. Nevertheless, even though synthetics are MUCH less prone to burn-off than are petroleum oils, there is still some burn-off during extremely high temperature operation.
Thus, it becomes important to discuss the manner in which petroleum and synthetic oils burn off. As a refined product, petroleum oil molecules are of varying sizes. Thus, as a petroleum oil heats up, the smaller, lighter molecules begin to burn off first.
Since the ash content in many petroleum oils is higher than synthetics, deposits and sludge are left behind to coat the inside of your engine. Detergent and dispersant additives are used to keep these deposits to a minimum, but only so much can be done. Unless you're changing a petroleum oil every 2,000 to 3,000 miles some deposits are going to be left behind.
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, on the other hand, because they are not purified, but rather designed within a lab 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 nearly 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 and little if any impurity. 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.
WHAT ABOUT THE ARCTIC FREEZE?
This is an issue that some people really don't think about when it comes to oil changes. Most people understand that at cold temperatures, an oil tends to thicken up, and many people know that synthetics do a better job of staying fluid. However, many people don't realize why petroleum oils tend to thicken up. More importantly, though, they don't realize that this thickening process can wreak havoc on their oil.
You see, because most petroleum oils contain paraffins (wax), they tend to thicken up considerably in cold temperatures. Therefore, in order to produce a petroleum oil that will perform adequately in severe cold temperatures, additives called pour point depressants must be used in high quantities. These additives are designed to keep the wax components of a petroleum oil from crystallizing. This maintains decent flow characteristics in cold weather for easier cold starts.
In areas where the temperature remains below zero for any period of time, these additives are used up very quickly because petroleum oils are so prone to wax crystallization. As a result, the oil begins to flow less easily in cold weather temperatures. Of course, the result is harder cold starts and tremendously increased engine wear. Thus, the oil must be changed in order to provide the cold weather engine protection which is necessary.
Synthetic oils, on the other hand, contain no paraffins. Therefore, they need NO pour point depressant additives. In addition, even without these additives, synthetics flow at far lower temperatures than petroleum oils. For instance, very few petroleum oils have pour points below -30 degrees F. Many synthetic oils, without any pour point depressants, have pour points below -50 degrees F. That's a big difference. There is, in fact, one oil on the market that has a pour point of -76 degrees F.
Since synthetics do not have any pour point depressants, there is no chance of these additives breaking down or being used up over time. There are no additives to break down. Therefore, synthetic oils maintain their cold temperature flow characteristics for a very long time. As a result, there is one less reason to change the oil if using synthetic as opposed to petroleum.
In addition, another part of cold weather driving that is extremely tough on an oil is condensation. Because it is so cold, it takes a fairly long drive to get the engine warm enough to burn off the condensation that occurs inside the engine. As a result, vehicles routinely driven short distances in cold weather will build up condensation within the oil. If left to do its dirty work, this water would cause acids to build up within the oil and corrosion would begin within your engine.
So, there are additives in the oil which are designed to combat these acids. Generally, the TBN value of an oil will be a good determination of how well and for how long an oil will be able to combat these acids. Most petroleum oils have TBN numbers around 5. Most synthetics have TBN levels over 8 or 9. Premium synthetic oils (especially those designed specifically for extended oil drains) will have TBN numbers around 11 to 14. This allows for much better acid control for a much longer period of time, thus decreasing the need for an oil change due to cold temperature condensation.
---------------------------------------------------------------- Excerpted with Permission from "The Motor Oil Bible" eBook Copyright 2000 Michael Kaufman ("The Motor Oil Bible" Author) Download it from: http://www.motor-oil-bible.com ---------------------------------------------------------------- Part 1 Are Oil Changes Really Necessary? - Part 3: What About Additive Depletion?