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 a technical specification referenced by most oil manufacturers which is an indicator of heat tolerance.
The lower the flash point of an oil the greater tendency for that oil to suffer vaporization loss at high temperatures and to burn off on hot cylinder walls and pistons. This leads to oil thickening and deposit build-up on critical engine components.
So, the higher the flash point the better. 400 degrees F, in my opinion, is the absolute MINIMUM to prevent possible high consumption and oil thickening due to burn-off. Higher would definitely be better.
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. Synthetics are up to the task. Petroleum oils are better than they used to be, but can still be a little overmatched. Nevertheless, even though synthetics are MUCH less prone to burn-off than are petroleum oils, there can still be a small amount of burn-off during extremely high temperature operation.
Since some motor oil “burn off” will occur whether using synthetic or petroleum oil, it becomes important to discuss the manner in which petroleum and synthetic oils burn off. Because it is a refined product and the refining process can only do so much, petroleum oil molecules are of varying sizes (the very smallest and largest of which are removed during the refining process – what is left is a smaller range of molecular sizes, but there is still significant variance in molecular size from one to another). Hence, as a petroleum oil heats up, the smaller molecules begin to burn off faster and easier than the larger molecules.
Moreover, some of those smaller molecules are actually contaminants that were left behind from the refining process. You see, when crude oil comes out of the ground it is a conglomerate of many different molecule types, a large portion of which are not useful for lubrication purposes at all. The refining process is designed to remove as much of those contaminant molecules as possible, but only so much can be done without raising the cost considerably.
As all of these smaller molecules burn, deposits and sludge are left behind to coat the inside of your engine which, obviously, reduces the efficiency of your engine and can also lead to greater heat build-up in your engine. Anytime heat increases beyond what is normal for an engine, longevity will suffer.
== Need A Heating Blanket for Your Engine?
Obviously, that would be silly, but, in some respects this is exactly what petroleum oils create inside your engine. You see, the larger molecules within your oil do not flow nearly as easily as the smaller molecules do (consider the difference in “pourability” of a heavy weight oil vs a light weight oil). Because of this, the larger molecules tend to be pushed to the “outside” of the oil stream, collecting at the surface of your engine components while the smaller molecules flow down the center of the oil stream between the larger molecules.
Thus, the larger molecules tend to “blanket” the components of your engine, trapping the heat there as opposed to pulling the heat away into the oil stream. Of course, this only exacerbates the heat problem as friction builds-up. As smaller molecules within the oil burn off, the larger, heavier petroleum oil molecules are all that is left to protect the engine. This makes the “heating blanket” effect even worse.
In contrast, synthetic oils, because they are not refined or purified from crude oil, but rather are designed within a lab for lubrication purposes, are comprised of molecules of uniform size and shape. Only the molecules that are useful to the purposes of lubrication and cooling are present within the oil, and only the molecule size that is desired for the particular viscosity being manufactured.
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. No oil thickening. No “heating blanket” effect.
Moreover, synthetics contain few, if any, contaminants as compared with petroleum oils since they are not a refined product. As a result, if oil burn-off does occur, there are few, if any, contaminants left behind to leave sludge and deposits on engine surfaces. Obviously, this leads to a cleaner burning, more fuel efficient engine.
In addition, because the molecule sizes are so uniform in a synthetic oil and there is no “heating blanket” effect, synthetics do a much better job of “cooling” engine components during operation. A cooler running engine means longer engine life. AND, oil temps will often be 10 to 30 degrees cooler than with petroleum oils because the heat dissipates so much better with a synthetic. Cooler running oil means longer oil life and better protection.