A Short Guide to Motor Oil and Additives

10 Apr, 2024 Automotive News Technical Articles Permalink

By Pat Lannone

            All motor oils, conventional, synthetic blends and fully synthetic contain various additives to make it perform better, motor oils also contain additives in different values which help prevent engine wear.

            Oil changing with oil filter replacement is a very important car maintenance item. It is advisable to choose the one that is correct for the type of engine in your car by consulting the manufacturer’s instruction manual. Using the wrong oil can cause damage to your engine and could void the warranty, if it is a new or rebuilt engine. The best guide to making the right choice is to ensure the American Petroleum Institute Certified (API) Starburst stamp is visible on the motor oil to be used. The API donut also helps you by indicating the performance of the motor oil, and its viscosity at different temperature levels or in other words, the Society of Automotive Engineers (SAE) rating. Today’s performance oils will also conserve fuel.

Understanding viscosity

            When choosing motor oil for your car, the most important consideration to keep in mind is its viscosity or the thickness of the motor oil. Motor oils that are too thick will not flow freely and cannot move through all the moving parts of your engine. They cannot effectively protect your engine from friction, contaminants and overheating. However, if the motor oil is too thin, it will not be able to coat the engine properly and is also ineffective.

            Ambient temperatures, the temperature which surrounds an object, can also affect the thickness and fluidity. Motor oils tend to thicken at lower temperatures and thin when the temperature rises. As a result, starting up the engine in winter can be more difficult and also consumes more fuel to reach the optimum temperature for combustion. Accordingly, manufacturers include additives in the motor oil so that it remains stable and continues to perform well. The figure you see on the API donut indicates the viscosity of the motor oil in winter and also, its fluidity after warming up sufficiently. The different additives have various properties, each contributing to the efficient working and life of your car engine. The SAE system for grading motor oils according to viscosity; the suffixes (0, 5, 10, 15 and 25) followed by the letter W designate the engine oil’s “winter” grade. Look to your owner’s manual. It specifies the viscosity grade required for your car’s engine. Today, the most common grades are 5W-30.

Types of motor oils

  • Conventional motor oils: If you have just bought a new model car and intend to use it for commuting short distances and light duty, where the engine is not likely to heat up excessively, then conventional motor oil will work very well in it. These oils have chemical additives and are available in a range of viscosity levels. If you’re using conventional oils, you’ll need to change the oil and filter in your car every 4 months or 4000 miles.      
  • Synthetic motor oils: Synthetic oils undergo strict quality control tests and are effective at both hot and cold temperatures. They do not evaporate easily, have a better viscosity index (the rate of viscosity change with temperature), and protect the engine against contaminants and overheating. Synthetic oils have a higher than usual percentage of additives and are considered high-quality motor oils. Some car manufacturers recommend that you use them.   
  • Synthetic blend motor oils: Synthetic blend oils include a combination of synthetic and organic oils. They are specially designed to protect your engine from oxidation, overheating and are ideal for use in heavy-duty vehicles that frequently carry loads, operate on uneven terrain, and tow trailers. These oils also help with fuel conservation.
  • High mileage oils: High mileage oils are designed for older vehicles that have seen long mileage or usage. These oils include additives that maintain its viscosity better and protect the engine from wear. They typically have a high viscosity and seal conditioners to help protect the bearing seals. In this way, they work to prevent oil leaks and restore the compression in the engine combustion chamber

Additives in motor oils

            Motor oil typically contains a Performance Additive Package that can make up 15% to 25% of its composition. The remaining percentage is base oil. The additives help to enhance the performance and viscosity of the motor oil. Oil manufacturers design their products to comply with the specifications of car companies and engine manufacturers. The base oils can vary themselves and assist in managing particulates and oxidation.

  • Viscosity-index additives: The Viscosity Index indicates the capacity of the motor oil to remain stable despite temperature fluctuations. The polymer additives ensure that the motor oil maintains its viscosity over a diverse range of temperatures.
  • Dispersants: These additives absorb and hold solid contaminants within the oil so that they do not damage the engine. In this way, they delay the forming of sludge, acids and varnish.
  • Detergents: These additives act on the piston-ring area, piston under-crown and other surfaces that overheat. They prevent the forming of deposits, rust and corrosion; and are used up over time.
  • Anti-wear agents: Anti-wear agents work to protect parts of the engine that are susceptible to high temperatures such as cylinder walls, piston rings, lifters and cams. These additives form a protective layer on these components and prevent the friction that can occur because of metal-to-metal contact. They also act as antioxidants and are used up over time.
  • ZDDP: ZDDP (zinc diakyl-dithiophosphate) is zinc and phosphorus compound that until recently formed a component of anti-wear additives. However, because of its propensity to damage the catalyst in converters. Even so, older vehicles that have a flat-tappet camshaft or an altered engine with higher-than-stock spring pressures need special motor oils that include this compound. The ZDDP provides the necessary lubrication to the cam lobes, lifter bodies, lifter bores and other engine parts. SL-rated oil contained 1,000 ppm and higher and SM rated oil with their limited 800 ppm and lower.
  • Friction modifiers: Graphite or molybdenum is a friction modifier that helps lower friction when the engine is operating under high temperatures and heavy loads. These additives also help you conserve fuel. They lose their effectiveness over the life of the motor oil.
  • Antioxidants: Under conditions of high temperatures in the engine, the motor oil can oxidize when reacting with oxygen. If that happens, the motor oil ages quickly, thickens and forms sludge. Antioxidants work to slow down this oxidation and the forming of deposits. They also help to keep the engine clean and prolong the life of the motor oil.
  • Anti-foam additives: If the motor oil foams and forms bubble, it is unable to coat all the important parts of the engine and keep it cool. As a result, it can cause serious damage to your engine, especially under high temperatures. Anti-foam additives prevent air from getting compressed in the oil and the forming of foam even if there is an excess of oil in the crankshaft because of accidental overfilling. In case of engines that have variable camshaft timing, the motor oil must also act as hydraulic fluid. Aerated motor oil will be unable to provide this function.
  • Rust and corrosion inhibitors: The internal parts of the engine can rust and corrode because of exposure to moisture and acids. These additives create a film over the parts and protect them against damage from these hazards.
  • Pour-point depressants: These additives work to prevent the hardening of wax particles in the motor oil under cold conditions. As a result, the motor oil flows freely even at low temperatures and does not need the engine to work harder to pump it. In this way, the engine continues to function effectively despite fluctuations in the outside temperature.
  • Dexos: Dexos engine oil is specially formulated engine oil created by General Motors engineers. It is used specifically in GM engines and works to provide a better mileage reduce emissions and prolong the life of the emissions system. It has better anti-foaming additives and needs changing less frequently as compared to other motor oils. The Dexos motor oil has better abilities to prevent the buildup of sludge and also helps protect the catalytic converter from damage.

API Engine Oil Service Category Charts – Gasoline Engines

Oil API GradeZinc (ppm)Phosphorous (ppm)StatusService Gasoline Engines
SN400-750400-700Introduced in October 2010Introduced in October 2010 for 2011 and older vehicles, designed to provide improved high temperature deposit protection for pistons, more stringent sludge control, and seal compatibility. API SN with Resource Conserving matches ILSAC GF-5 by combining API SN performance with improved fuel economy, turbocharger protection, emission control system compatibility, and protection of engines operating on ethanol-containing fuels up to E85.
SM400-870400-800Introduced on 30 November 2004Category SM oils are designed to provide improved oxidation resistance, improved deposit protection, better wear protection, and better low-temperature performance over the life of the oil. Some SM oils may also meet the latest ILSAC specification and/or qualify as Energy Conserving. They may be used where API Service Category SJ and SL earlier categories are recommended.
SL1000-14001000-14002001 Gasoline Engine ServiceCategory SL was adopted to describe engine oils for use in 2001. It is for use in service typical of gasoline engines in present and earlier passenger cars, sports utility vehicles, vans and light trucks operating under vehicle manufacturers recommended maintenance procedures. Oils meeting API SL requirements have been tested according to the American Chemistry Council (ACC) Product Approval Code of Practice and may utilize the API Base Oil Interchange and Viscosity Grade Engine Testing Guidelines. They may be used where API Service Category SJ and earlier categories are recommended.
SJ1000-14001000-14001997 Gasoline Engine ServiceCategory SJ was adopted in 1996 to describe engine oil first mandated in 1997. It is for use in service typical of gasoline engines in present and earlier passenger cars, vans, and light trucks operating under manufacturers recommended maintenance procedures. Oils meeting API SH requirements have been tested according to the American Chemistry Council (ACC) Product Approval Code of Practice and may utilize the API Base Oil Interchange and Viscosity Grade Engine Testing Guidelines. They may be used where API Service Category SH and earlier categories are recommended.
SH1000-14001200-1400ObsoleteFor model year 1996 and older engines
SG1000-14001200-1400ObsoleteFor model year 1993 and older engines
SF1000-14001200-1400ObsoleteFor model year 1988 and older engines
SE1000-12001200-1400ObsoleteFor model year 1979 and older engines
SD1000-12001200-1400ObsoleteFor model year 1971 and older engines
SC1000-12001200-1400ObsoleteFor model year 1967 and older engines
SB1000-11001200-1400ObsoleteFor model year 1963 and older engines
SA00ObsoleteFor model year 1930 and older engines

Valve Rocker Lifter Cam Diagram

          1 – Valve 2 – Rocker Arm 3 – Valve adjustment screw 4 – Valve adjustment locking nut 5 – Push rod 6 – Hydraulic and non-hydraulic lifter 7 – Flat tappet camshaft and cam lobe