The viscosities and characteristics of oil vary between brands and base stock origin. In the early 1900s, consumers needed some way of classifying what oil would work best for their engine. They needed some recognizable standard. To answer that need, the American Petroleum Institute (API) and the Society of Automotive Engineers (SAE) standardized were born.
What do SAE numbers mean?
SAE ratings measure the viscosity, or thickness, of oil at a constant temperature. Thin oil flows more easily (it can even be close to the consistency of water) and has a lower viscosity number, while thicker oils have a higher number and are more resistant to flow (such as molasses or honey at room temperature).
||Viscosity @ Room Temp (mPa-s)
|SAE 40 motor oil
|SAE 30 motor oil
What is "Multi-weight" oil?
- This viscosity is then expressed in the amount of time taken for a standard quantity of oil to pass through a standard orifice at a given temperature (measured by a viscometer).
- The thicker (higher viscosity) of oil, the slower it will flow, commonly measured in kinematic and absolute (centistokes - cSt) values.
- Higher values mean more viscous fluids.
"Multi-weight" ratings indicate viscosity at both cold and hot operating temperatures. For example, SAE 10W-30 tells us both low temp and high temp viscosity. The "W" stands for "winter" (not weight), so receives a viscosity rating of '10' at colder temperatures. It conversely receives a rating of '30' (a thicker, more viscous consistency) at higher temps.
The viscosity index improver (see "Oil Additives") produces a thickening effect at high temps but is dormant at low temps, thereby producing "multi-grade" oil. Exposed to extreme conditions, these improvers can "shear" and lose their effectiveness.
Most oils will meet the viscosity requirements of at least one of the "W" grades. With the right quality base oil and proper refining techniques, the finished product needs no additives to improve cold temperature performance.
Nevertheless, some receive a "straight weight", or single grade SAE classification, indicating that product has been formulated only for high temperatures. SAE 30 is one example of a single grade oil.
What is an 'API rating?'
As the oil industry began innovating new products and additives to enhance performance, the need increased to classify each oil accordingly. In 1924 the API answered the call by establishing a grading system for both gasoline and diesel engine oils. They did so to help end users know what to expect from their oil's performance and quality. API's classification does not point to oil viscosity (see the "SAE Viscosity Grades" section), but primarily to the automotive industry's drive toward fuel efficiency and lower emissions standards.
The automotive category of the API rating system shows the "S" service grade, while the "C" category meets requirements suited to diesel engines.
Each progressive suffix indicates the latest API oil standard. Starting with "SA" (straight mineral oil) in the 1930s, API standards moved through the alphabet, skipping "SI" and "SK" from the sequence.
As auto makers came under increased pressure to develop more fuel efficient vehicles, oil formulations were changed to accommodate. Today, the latest API rating is "SM", with "SN" soon to follow.
For engines designed for use in lawn mowers, the highest API rated oil may not always be best. The same additives which enhance fuel economy for your car may do nothing to protect from the severe abrasion and corrosion exposures inherent in the life of your lawn mower. See "Oil Additives" section for more guidance on the most common components used to get the best performance from your machine.