Ratting such as the API and ACEA standards refer to the oil's quality. For grade , SAE ( Society of Automotive Engineers ) is the organization that grades engine oils according to their kinematic viscosity rating that describe the oil's function and viscosity standard.
( Association of European Car Manufacturers ) standards have three categories : A - for petrol engines . B - for diesel engines and E for commercial diesel engines.Levels of lubricants performance are expressed by a following number :1 for the fuel-economy,2 for standard performance and 3 for high performance and / or extended drain. Example : A1 describes fuel-economy petrol engine oil, A3 describes high-performance petrol engine oil.
( American Petroleum Institute ) oil performance standard is expressed by two latters. The first letter indicates
whether the oil os for petrol (s) with `s' ( meaning Service category) or diesel (c) with `C' ( meaning Comercial category ) engines. The second letter special the level of technical performance within the relevant group : A for the minimum level for both petrol and diesel, and J (petrol) and F diesel for thr highest level. High performance engine oil must meet API SL standards which is indicative of the oils having standard levels of anti-wear additives, detergents and dispersants. `SM' is the current top grade, which recently replaced `SL' `SM' is basically an upgrade version of the `SL' oil that has been tested more vigorously.
There are two general grades of motor oils i.e single grade and multi -grade. Single grade oils are usually used when the operating temperature is relatively stable ( or for specific applications), meaning they cannot meet both winter and summer requirements. For applications where the temperature range is not very wide, such as in lawn mowers, single grade oils will suffice.
Multi -grade oils are made by adding viscosity index(VI) improver additives (oil-soluble organic polymer )to single-weight oils to improve th temperature range. VI is simply the measureof oils thickness and ability ti flow at certain temperatures when it is subject to heat. The benefits of adding VI improvers are the reduce viscosity
changes with temperature , reduce fuel consumption and allow easy cold starts. These oils are thin enough to work at cold temperature and safe enough to work at high temperatures ( 212 degrees test point ).
Multi-grade are given two numbers-the first number represents the base stock weight followed by the letter W (winter). "W" grade oils are defined by their ability to flow and lubricate at cold start up, 0W (-35 C) ,5W (-30C),10W (-20C), 15W (-15C),20W (-5C), the lower (low viscosity) this number, the more fluid (thinner) at low temperature , the easier th engine start. The second number represents the high temperature viscosity rating :30(+30C), 40 ( +40C) and 50 (+50C). The higher ( high viscosity ) this number , the less the oil will thin when hot, which means it will hold without breaking down at high temperatures.
The thicker an oil is, the higher its viscosity and vice versa. High viscosity oil would get in cold temperature if additives are not added to thinner single-weight base stock for good low temperature pour performance.
A VI improver is akin to a very long string which at low temperatures is tightly coiled into a small ball that floats in the oil with little effect on th oil's viscosity . However, at high temperature this ball expand, uncoils and encloses a larger volume of oil within its structure. This effectively prevents the oil from thinning because of the greater volume taken up by the expanded balls if VI improver molecules.
Oils with exceptionally high viscosity rating have more internal friction and tend to cause the oils to pump up and the engine become nosier. They may not pump to the proper parts of your engine especially at low temperatures, and the film may further tear at high RPM. Older engines need to use thicker oils such as SAE 50 because of the wear and tear of the engine.
Oils with exceptionally low viscosity (generally SAE 0W - 30) may lose its film strength it higher temperatures however ; the losses caused by friction internal to the lubricant are reduced; the petrol consumption is therefore enhanced by a few precent. Low viscosity oils such as SAE 20, 30, or 40 are thinner and reach engine parts faster on cold starts. However caution should be used as this type of oil is not suitable for all engines. Motor oils must also cater for thin film lubrication in areas of the engine where thick films can't form because the relative speed of moving parts os too slow for oil wedge formation and or because the loads are too high.
New cars perform better and get good fuel economy with API SH grade thinner 5W-30 or 10W-30 oil, thereby avoiding the possibility of ring sticking and other problems that can occur in oils loaded with VI improvers. For NA streets cars, thinner oil's low temperature flow characteristics help avoid significant wear at startup , which is much more important than a little more oil pressure in a hot engine . This is necessary in Overhead Cam Engines with cam bearing located above the oil supply. When you start your engine , let it idle for about 15 second to warm before moving off.
Choose the narrowest span with a heavier base if your temperature zone allows, a wider span means more polymers which is not good for your engine.For example 10W -30 has a narrower span than 10W-40, thus requires fewer polymers. 10W-40 required more polymers than 10W-30 to work at hot temperature. Don't make a choice of a lighter base if there's no need in the same name of temperature. For example 10W-40 has the same 30 point spread with 20W-50, but 20W-50 has a 20W base which is heavier than 10W, thus 20W-50 oils needs fewer polymers than 10W-40.
Forced induction engines such as turbo bearing can heat engine oil to the point of breakdown, Forced induction cars would therefore have dedicated oil coolers to mitigate this. Race oils are always high viscosity that is why it is hard to justify anything but synthetics due to their wider latitude of engine protection under heavy loads especially in our hot climate.