Modern Lubricants for Automotive Applications

Diesel Fuel Article

If this site has helped you consider a Donation. Donation Info

Because we all have substantial investments in our Hummers and depend on them to perform under diverse conditions, care and maintenance is always an important issue. The essence of a Hummer is it’s engine and drive train. Because of it’s all independent suspension, full-time four wheel drive, torque biasing axles and geared hubs, lubrication is extremely important. The transmission is not an issue because it requires Dexron III automatic transmission fluid. I recommend using synthetic ATF in the transfer case because it operates at a very high temperature due to it's design. We do have a choice for the engine, differentials and geared hubs as long as you use a product as good or better than AM General requires. I started thinking about synthetic oil when the weather started to get cold (I live north of Chicago). I figured that gas mileage and performance would be further hindered by all of that cold thick gear oil in the axles and hubs. This was a perfect opportunity to remedy my curiosity about synthetic oil.

After reviewing the following information and years of experience the conclusion I came to was that you can change the axles and geared hubs to synthetic oil on a newer truck (low miles) but it will give you little benefit unless you are in a very extreme climate. Since none of the gear oil requires periodic changes the cost is minimal. Synthetic oil will flow at very cold temperatures and not break down in the heat under load. It only takes 6 quarts for the whole job. These oils are 75-90 GL-5 rated.

On trucks with more then 65,000 miles or so I would use regular gear oil or Lucas Hub oil because older hub seals tend to leak with the thinner synthetic. I used the Lucus hub oil and frankly, it didn't give me any different wear on the hubs and seals compared to regular gear oil. The stuff is thick so on hubs with leaky seals it might give you a little leeway in changing a leaky seal. My feeling is that if it's leaking, fix it.

The hardest part of the whole job is removing the wheels to get to the hubs, (you don't really have to but it's easier) so change the hub oil the next time you rotate your tires.

I didn’t go to synthetic oil for the engine because the Hummer has a large oil cooler which keeps the oil from overheating and breaking down. Under normal use, changing the oil and filter every 3,000 miles should be just fine. Synthetic oil would be a good option for those that use their trucks in very severe service or when its below zero for an extended period. The black soot that builds up quickly in the oil of an indirect injection diesel engine like ours will increase wear if not changed about every 3000 miles. A point of interest; the reason that new Duramax and Cummins diesel pickups can go for 7500-10000 miles between changes is that they hold 10-12 quarts and are a direct injection design which puts out less soot. This means that the oil stays cleaner longer.

When changing oil every 3000 miles, the cost of synthetic makes it a poor choice. I personally use Rotella-T oil which is available everywhere. look on the shelf at any truck stop and you will see this is the most popular oil truckers use in thier diesels which go well beyond 500,000 miles.

There's been a great deal of interest, of late, in the performance of synthetic lubricants. Manufacturers have enticed the motoring public for a number of years now, with claims for increased fuel economy, reductions in friction and wear, decreased oil consumption, better cold cranking performance, and extended drain intervals. Many motorists however, remain skeptical, as the price of synthetics is usually much higher than conventional petroleum based oils. In addition, a great deal if misinformation has circulated regarding them. Are these claims simply hype, or is there something here that the average motorist can be interested in? Let's take a look.

Synthetic lubricants have been around for a long time. Synthesized compounds are the only thing that will continue to flow at the low temperatures found in the arctic or in outer space. During the past twenty years, some of these same benefits have been made available to the general public.

As usually stated in engineering texts, and intuitively grasped by most laymen, a lubricant is inserted between two moving surfaces to reduce friction, and the resultant generation of heat and wear. Additionally, the lubricant is used for Removal of heat (up to 1/3 of combustion heat may be transferred away from engine by oil), containment of contaminants and sealing. "Hydrodynamic" lubrication, which is what you want, exists when two surfaces are separated by a relatively thick film of lubricant.Take out a deck of cards. Place the deck on a table, and with your hand, move the deck horizontally. Notice how the bottom card does not move, while the top card moves the most. What's happening here is that the friction between the table and the bottom card keeps it from moving. The type of stress you applied to the cards is called a "shear stress", and is equal to the horizontal force you applied to the top card. If you drop a steel ball into a glass of molasses (a high viscosity fluid) it will drop slowly because of the internal friction of the fluid. Likewise, dropping the same steel ball in a glass of water, will cause it to drop rapidly because the fluid does not have a particularly high viscosity. What this simply means is that viscosity is a measurement of the internal friction of the fluid, and its resistance to motion.

One of the problems with this internal friction is that it produces heat. The viscosity of a fluid generally decreases with temperature, and increases as the temperature drops. Also, the viscosity of any fluids, engine oils among them, drop with high shear rates. If the lubrication film is too thin, as the viscosity decreases, the lubricant is less able to withstand the loads placed on it. Heat is generated, reducing the viscosity even further. Surface to surface contact may occur. In thin films, the friction between the two surfaces actually increases as the viscosity decreases. Such films are termed "unstable". It is essential then to provide a film which is sufficiently thick to provide proper lubrication. The machine whether it be an engine, transmission etc. must be designed in such a way as to prevent the formation of a thin film. In a good design the viscosity has to be high enough to prevent metal to metal contact at high temperatures while allowing the lubricant to flow at lower temperatures. Proper use of materials, clearances, finishes and recirculating flow systems found in most engines achieve this end.

If the film is thick enough the friction between the mating surfaces actually goes down as the viscosity of the lubricant drops. The temperature drops, and the viscosity of the fluid rises slightly. This acts as a stabilizing effect, and prevents loss of film thickness.

So what about the lubricant itself? What kind of specifications does it have to meet? The American Petroleum Institute (API), the American Society for Testing Materials (ASTM), and the Society of Automotive Engineers (SAE) have cooperatively developed specifications for lubricating oils. If you take a look at the top of a motor oil can, you'll find the following: SAE viscosity specification (such as 5W30, which means that it is a multivis oil that meets both the 5W and 30 specifications.), an API service classification (such as SF/CD), and perhaps an "energy conserving" designation. The SAE viscosity designation, means that the oil meets SAE J300 specifications for cold cranking (if a "W" rated oil) and at 100 degrees Celsius (if without a "W" rating), when proper ASTM testing procedures are followed.The API service classification is a bit more complex. You see, an oil may initially meet the SAE viscosity specification, but when run at high temperatures for a period of time, its performance may deteriorate. The API classifications for most engine oils are set for spark ignition engines (such as SF, where the "F" is a chronological designation), and compression ignition (diesel) engines (such as CD). Several test sequences are run using a standard engine. For instance, rust and number of stuck lifters are rated, the viscosity increase over time at, say 100 degrees F is measured, and the amount of sludge, varnish, oil screen clogging, and cam lobe wear is estimated or measured.

Just what does an oil consist of, and how can it be compounded to meet these specifications? Let's look first at conventional oils. Crude oil as it comes from the ground is made up of a number of hydrocarbon compounds primarily paraffins, but it also includes other compounds. Often, these compounds are separated by viscosity through a distillation process. Since different fractions of the crude have different boiling points as well as different viscosity, progressive boiling is used. Those fractions with lower boiling points are allowed to vaporize, and are collected and then cooled. These neutral fractions typically have lower viscosity, while the bright stocks (those with higher boiling points) generally have higher viscosity.

But here's a problem. If we compound an oil to have a relatively low viscosity (or a multivis oil with a significant amount of these lower boiling point/lower viscosity stocks) some of them will vaporize at high temperatures, resulting in higher oil consumption. What's left behind has a higher viscosity. Varnish and sludge are also present. If the decrease in viscosity, amount of sludge, varnish, and cam lobe wear are too high, it fails the API service test.That's why a 5W30 oil that meets the SF rating represents a major step. Those oils are said to be "energy saving" since their lower viscosity at lower temperatures results in lower part to part friction. Yet by passing the SF rating, it shows that it's still pretty good.

Now, there are many things in the average motor oil than various refined fractions of crude. Included are various additives, such as anti-wear agents, extreme pressure (EP) additives, antirust agents, corrosion inhibitors, detergents, dispersants, pour point depressants, viscosity index (VI) improvers, seal swell agents and friction modifiers. Finally, an oil company may add various compounds which help protect the base stock, such as antifoam agents, antioxidants, and metal deactivators. Most of these are self explanatory. They are added to enhance the performance of an oil. The EP additives are put in to help the oil hold up between surfaces which feature high contact stresses such as those between the cam lobes and followers. Detergents and dispersants are put in to help remove dirt and sludge and hold it in suspension, until it's either removed in the filter, or the oil is changed. Pour point depressants are added to inhibit wax crystal growth at low temperatures. This gives the oil better cold cranking performance. The antioxidants are important as they prevent the oil from reacting with oxygen at high temperatures and forming sludge, varnish, and lacquer.

VI improvers are designed to help an oil's viscosity/ temperature performance. The viscosity index (VI) is a measurement of how an oil's viscosity changes with temperature, compared to reference oils. The higher the number, the better. VI improvers are polymer compounds with interlocking structures (polymers are long chain molecules). Because these chains are interlocked, they don't move as easily at high temperatures and resist viscosity loss. Unfortunately, they don't necessarily contribute anything to lubricity, and in fact begin to wear out under shear stresses. As they wear, the oil's VI deteriorates, and we're left with the VI improver, which has to be held in suspension as waste. This is another reason to change your oil frequently! The VI improvers sensitivity to high shear stress is significant in that if the shear stress is high enough, the oil may experience either a temporary or permanent loss of viscosity!

So where do synthetics fit in? What are they? The term "synthesize" means to put together from small bits. Rather than separating crude into various fractions as is done with conventional oils, synthetic base stocks are made by reacting various organic chemicals together. For instance, if an acid and a alcohol are allowed to react, a compound known as an ester is produced. Other synthetic hydrocarbon compounds are also suitable for lubricating oils, and manufacturers may blend two or more compounds together to arrive at suitable properties.

A synthetic may require considerably less VI improver then conventional oil to have the same viscosity index. Remember that the VI improver wears out. Synthetic's are also more thermally stable. Synthetic Oil does not evaporate as readily and has 10% better heat transfer than Petroleum based lubricants. Synthetic base stocks also have lower pour points often below 50 degrees F, and require little or no pour point depressant. For instance, synthetics can be compounded with very low pour points. This gives good cold cranking performance. They may also be compounded with slightly lower viscosity at lower temperatures (while still meeting SAE specifications). This helps to reduce friction, and results in less wear, and better fuel economy.

Now the 5W30 "energy saving" oils will do the same thing, but as we've discussed before, to lower the viscosity, these oils may be compounded with fractions which have a higher volatility. After a period of time, they begin to boil off or oxidize, leaving behind an oil of higher viscosity. Now, that same oil may meet API SF specifications, but a synthetic may remain stable for a LONGER period of time. That means that longer drain intervals are possible.

A word on use. Some synthetic compounds are not compatible with conventional oils. However, most manufacturers, have recognized that one may add a quart of their product to someone else's, and have compounded them to be. Try to avoid mixing conventional and synthetic oil. While they are compounded to be compatible, the performance may not be the same when mixed together. It's ok in a pinch, but I don't make a habit of it. Also, the lower friction resulting from the use of a synthetic lubricant makes them unsuitable for break in.

Gear Oil

Gear oil viscosity is measured at 150 F vs. 210 F for motor oil. Therefore, 40 W motor oil is the same as 95 W gear oil. Gear oil is acidic, motor oil is alkaline. Gear oil needs very high wear protection Extreme Pressure (marked as EP). Therefore, it has a very high sulfur and phosphor content. Sulfur and Phosphate reactions start at a lower temperature, and Gear Oil has much more additive than motor oil. This additive is corrosive to copper bearings and bronze synchro rings. Gear oils decompose at lower temperature, usually 250 F. Gear mesh in Gears literally chops up and cuts apart the long polymer chains of Viscosity Index improvers. Hypoid type gear sets have a sliding rather than rolling action, and therefore require much greater wear protection. Gear lubrication ratings go from GL1 through GL6. GL4 is suitable for light duty hypoid sets. GL5 (HUMMER) has lots of sulfur, for heavy duty hypoids and used for heavy trucks and Tow Vehicles. GL6 is a heavier weight.

Slick 50 / STP ?

The experts all agree that it does not do anything. To plate teflon on a metal needs an absolutely clean, high temperature surface, in a vacuum. Therefore, it is highly unlikely that the teflon in slick 50 actually plates the metal surface. In addition the Cf (Coefficient of friction) of Teflon is actually greater than the Cf of an Oil Film on Steel. Also, if the teflon did fill in 'craters' in the steel, than it would fill in the honing of the cylinder, and the oil would not seal the piston rings. STP is nothing more than a VI improver.

One source of information in this article is a summary of a paper written by Richard G. Golembiewski, P.E. and notes from a lecture to Dema Elgin's High Performance Engine Class By Roy Howell, Chief Chemist, Redline Synthetic Oil Company.


You can send in samples of oil for testing to Blackstone Labs