By Roy Berndt, engine specialist
First off, all oil breaks down. That generally will include base stocks 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 are 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 base stocks and additives can last almost indefinitely with the right filtration system.
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° F is the absolute minimum to prevent possible high consumption.
Even the best petroleum oils will have flash points only as high as 390° and 440° F. Some actually have flash points as low as 350° F. For today’s hot-running engines, this is not nearly enough protection. Just about any synthetic you come across will have a flash point above 440°. Premium synthetics can have flash points over 450° F with some even reaching as high as 500° F. That’s a big difference.
It’s important to understand how petroleum and synthetic oils burn off. As a refined product, petroleum oil molecules are of varying sizes. So, 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 the 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 don’t flow nearly as well and tend to blanket engine components, which just makes the heat problem worse.
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 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.
Synthetics do a much better job of “cooling” engine components during operation. Because of their unique flow characteristics, engine components are likely to run 10-30° cooler than with petroleum oils. This is important, because the hotter the components in your engine get, the more quickly they break down.
Most techs understand that at cold temperatures, an oil tends to thicken up, and many techs know that synthetics do a better job of staying fluid. However, many 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.
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 that 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° F. Many synthetic oils, without any pour point depressants, have pour points below -50° F. That’s a big difference. There is, in fact, one oil on the market that has a pour point of -76° 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.
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. Consequently, 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 that are designed to combat these acids. Generally, the Total Base Number (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 TBNs around 5. Most synthetics have TBN levels over 8 or 9. Premium synthetic oils (especially those designed specifically for extended oil drains) will have TBNs 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.
It is true that the additives in many oils begin breaking down after only a few thousand miles. What needs to be recognized is that there are different quality “grades” of additives just as there are different quality grades of just about any other product that you buy. There are also different combinations of additives that tend to work for better and for longer when combined than when used individually.
Shear stable viscosity index improvers help premium synthetic motor oils maintain their viscosity in the range appropriate to each grade over extended drain use. Conventional oils formulated with easily sheared viscosity index improvers often drop out of viscosity specification relatively quickly sometimes even before the end of a 3,000-mile oil drain interval. Viscosity loss leaves oils incapable of protecting engines from metal-to-metal contact and wear in high temperatures.
Oil will be contaminated in three major ways.
One will be through debris that comes in through the air intake. Once it makes it through the air filter, it ends up in an engine’s oil. Once in the oil, it starts damaging the engine.
The second source of contamination will be metal shavings from the inside the engine. The lesser the quality of the oil, the higher percentage of these shavings because there will be more metal-to-metal contact inside the engine.
The third source of contamination will be from combustion byproducts. Combustion byproducts will generally raise the acidity of the oil, which causes corrosion in an engine. In addition, they will be left behind as the engine oil burns off and will collect on the inside of your engine as deposits.
To maintain the viability of the oil as well, as protection of the engine, the contaminants have to be removed/neutralized. And that, my friends, would be an article on filtration, and covered in a future issue.
Internal Contributors To Excessive Oil Consumption
Worn Valve Stems and Guides: When wear has taken place on valve stems and valve guides, the vacuum in the intake manifold will draw oil and oil vapor between the intake valve stems and guides, into the intake manifold and then into the cylinder where it will be burned. If this condition is not corrected when new piston rings are installed, an engine is likely to use more oil than it did before because the new piston rings will increase the vacuum in the intake manifold. When gum or deposits on the valve stems are removed, the seal previously formed will be removed and leakage will be more pronounced.
This is particularly true on overhead valve engines where loss of oil may occur on the exhaust valves as well as on the intake valves. Reaming the valve stem can frequently cure high oil consumption caused by too much valve guide clearance. In some cases new valves may also be required.
Worn Front or Rear Main Bearing Seals: Worn front or rear main bearing seals almost always result in oil leakage. This can only be determined when the engine is operated under load conditions. Bearing seals should be renewed when worn because a slight leak will result in extremely high oil consumption just as it would with an external oil leak.
Worn or Damaged Main Bearings: Worn or damaged main bearings throw off an excessive amount of oil, which flows along the crankshaft and is thrown up into the cylinders. The amount of oil throw off increases rapidly when bearing wear increases. For instance, if the bearing is designed to have 0.0015” clearance for proper lubrication and cooling, the throw off of oil will be normal as long as this clearance is maintained and the bearing is not damaged in any way. However, when the bearing clearance increases to 0.003”, the throw off will be five times normal. If the clearance is increased to 0.006”, the throw off will be 25 times normal. When the main bearings throw off too much oil, the cylinders are usually flooded with more oil than can be controlled by the pistons and rings. This causes burning of the oil in the combustion chamber and carboning of pistons and rings.
In a conventional, full-pressure lubricated engine, a large loss of oil at the main bearings may starve the downstream connecting rod bearings of lubrication to such an extent that sometimes, especially at low speeds, insufficient oil may be thrown on the cylinder walls. This will cause the pistons and rings to wear to such an extent that they will not be able to control the oil at high speeds. The effect of main bearing wear will be high oil consumption.
Worn or Damaged Connecting Rod Bearings: Clearances on connecting rod bearings affect the throw off of oil in the same proportions as mentioned for main bearings. In addition to this, the oil is thrown more directly into the cylinders. Worn or damaged connecting rod bearings flood the cylinders with such a large volume of oil that the pistons and rings, which are designed to control a normal amount of oil or a reasonable increase in the normal amount, are overloaded to such an extent that some oil escapes past them to the combustion chamber and causes high oil consumption.
Note: Insufficient bearing clearance can also produce piston, ring and cylinder damage as well as damage to the bearing itself.
Worn or Broken Piston Rings: When piston rings are broken or are worn to such an extent that the correct tension and clearances are not maintained, they will allow oil to be drawn into the combustion chamber on the intake stroke and hot gases of combustion to be blown down the cylinder past the piston on the power stroke. Both of these actions will result in burning and carboning of the oil on the cylinders, pistons and rings.
Broken rings are especially damaging because their loose pieces with jagged ends are likely to cut into the sides of the piston grooves. This causes land breakage, which results in the complete destruction of the piston assembly. Instead of reinstalling worn rings during engine overhaul, it is always advisable to replace them. New rings have quick-seating surfaces that enable the rings to control oil instantly, unlike rings that have been used in the past. Used rings, even those that have been only slightly worn will still have polished surfaces that will not seat-in properly and will lead to excessive oil consumption.