Today's lubricant technology has, without a doubt, improved the performance and reliability of modern day racing oils, but with so many choices and as many claims,choosing the best lubricant can be difficult. Petroleum-based oils vs. Synthetic oils has been a growing debate among professionals in racing for sometime now. Most opinions have been generated on experience with a particular product with very little research as to what a product is actually made up of and what it's capabilities are.
Let's take a look at the primary functions of oil and what some of the differences are between petroleum-based and synthetic-based oils.
Engine oil performs many functions in an engine to include:
1.Transmits forces to the crankshaft from the piston pin through the connecting rod bearing and through a very thin layer of oil. The forces on that oil can reach 147,000 psi, depending on the application.
2. Lubricates / Quiets components in relative motion by achieving metal separation with the absolute least amount of oil drag, especially during start-up.
3. Cools components by absorbing and removing heat generated by friction and combustion temperatures.
4. Cleans components by washing contaminants out away from the source and holding them in suspension.
5. Seals between the cylinder wall and piston rings during all cycles for improving performance and minimizing cylinder leakage.
6. Conditions seal/gasket materials against degradation and conditions metals from acidic corrosion.
The oils that can perform these functions the best are the ones best suited for high performance applications. Some of the key characteristics which determine an oil's ability to perform those functions are:
1. Film Strength- determines an oil's ability to separate metal under high stress.
2. Shear Strength- determines an oils ability to maintain it's proper viscosity or weight under extreme pressure.
3. Pour Point- determines the lowest temperature an oil is able to flow and remain fluid-like under defined conditions.
4. Viscosity Index- (VI) determines the oil's ability to maintain a consistent viscosity (or thickness) relative to temperature change. The VI Number is a mathematically derived number expressing this characteristic. The higher the VI Number the more resistant the oil is to viscosity change with temperature change
5. Thermal Stability- determines an oils ability to absorb and manage heat (oil temperature) under high temperature and high load conditions for measured periods of time without oxidizing.
6. Induction period- is the time that elapses before substantial changes occur in the oil (drain-life)
These key characteristics should be tested to ASTM Standards. These are the standards all materials should be tested to. Results from these tests will play a major roll in determining whether the oil can perform it's primary functions. The ASTM tests include the following:
1. D2670-95 Standard Test Method for Measuring Wear Properties of Fluid Lubricants (Falex Pin and Vee Block Method)
2. D6278-02 Standard Test Method for Shear Stability of Polymer Containing Fluids Using a European Diesel Injector Apparatus
3. D97 Standard Test Method for Pour Point of Petroleum Products
4. D2270-93 Standard Practice for Calculating Viscosity Index From Kinematic Viscosity at 40 and 100°C
5. D445-03 Standard Test Method for Kinematic Viscosity of Transparent and Opaque Liquids (the Calculation of Dynamic Viscosity)
6. D4742-02a Standard Test Method for Oxidation Stability of Gasoline Automotive Engine Oils by Thin-Film Oxygen Uptake (TFOUT)
Now that we have an idea about what an oil needs to do, what characteristics and oil needs to have and what standards those characteristics need to be tested to, we can take a look at what some of the fundamental differences between petroleum-based oils and synthetic-based oils are.
The first thing we should realize is that the base oil physically makes up approximately 70-80% of the finished product. The remaining 20-30% of the oil is made up of additives. The additives are designed to change the physical properties of the base oil and are classified according to their specific functions. They include:
1. Anti-wear (AW) additives
2. Friction Modifiers
3. Pour-point improvers
4. Oxidation and corrosion inhibitors
5. Viscosity-index improvers
6. Detergent and dispersant additives
7. Anti-foaming agents
The additive package in any oil will be a major determining factor in determining the quality of the finished product. This is what usually separates certain oil blenders from the rest. Assuming that all blenders have access and availability to the same additive ingredients, the oil will reflect which additives were actually used. The best product is usually the one where cost is not a consideration in terms of engineering and testing. When no expense is spared, the best ingredients can be utilized in formulating the finished oil and the results from testing will make it evident.
Petroleum-base oil is derived from crude oil in the ground. Certain desirable hydrocarbons are extracted and blended for lubricant use. Modern day semi-synthetic base oils now undergo a three step purification process that involves hydrotreating. The results from this process is a finished petroleum-based oil that has a much higher viscosity index and increased thermal stability.
Synthetic-base oil is derived from the basic raw material ethylene, a man-made hydrocarbon used in numerous chemicals. Linear alpha olefins (LAO) are derived from ethylene through conventional polymerization. Polyalphaolefins (PAO) are produced from the LAOs by means of catalyst technology. The PAOs are hydrotreated to produce a finished oil with exceptionally low pour points, increased anti-oxidative characteristics and a natural resistance to volatility.
The American Petroleum Institute (API) sets the industry standards for all commercially sold engine oils. In API publication 1509, an engine oil classification system and testing guidelines are laid out for oil blenders that are looking for API certification. A table for base oil stocks is also included in pub. 1509 in order to classify quality. It's as follows:
When comparing Group 3 and Group 4 base oils, the four critical areas to compare are:
1. Pour points
2. Low temperature viscosity
4. Oxidative stability
Pour points is an area where PAOs have a clear advantage. Group 3 oils of the same viscosity can have a pour point 40 degrees C or higher. Pour-point depressants have to be used just to approach the pour points of PAOs.
Low Temperature Viscosity is another area where PAOs have another advantage. Even though the viscosity index is very similar in both groups, the PAOs tend to resist low temperature thickening better.
Volatility or evaporation is an area where Group 3 and Group 4 base oils are very similar for a given viscosity grade. PAOs will perform better in this area however if the Group 3 base oil is not specifically distilled for it.
Oxidation and Thermal Stability is another area where the characteristics between the two groups are similar. Both base oils groups can be formulated to manage heat, but the PAOs tend to have a more natural resistance to oxidation and oil cooking.
In conclusion- because of the overall flow characteristics, the PAOs are the better choice for racing. These flow characteristics mean less wear and more horsepower which are made evident through detailed oil analysis and dyno testing. PAOs tend to need less additives to perform it's primary functions which equates to longer drain-life. A good point to remember is that a well formulated Group 1 based oil, which would require sufficient additives, can outperform a poorly formulated PAO based oil.