Motor oil does more than just lubricate an engine. It also forms a film on bearing surfaces that lifts and separates moving parts so they don't touch to reduce friction and wear. The oil film also acts like a shock absorber to cushion reciprocating and rotating parts. Oil also serves as a coolant for critical engine parts such as the crankshaft bearings and valve-train. Oil also helps prevent rust and corrosion inside the engine, and helps keep surfaces clean by dissolving and carrying away dirt and varnish deposits. UNDERSTANDING OIL VISCOSITY "Viscosity" refers to how easily oil pours at a specified temperature. Thinner oils have a water-like consistency and pour more easily at low temperatures than heavier, thicker oils that have a more honey-like consistency. Thin is good for easier cold weather starting and reducing friction, while thick is better for maintaining film strength and oil pressure at high temperatures and loads. The viscosity rating of a motor oil is determined in a laboratory by a Society of Automotive Engineers (SAE) test procedure. The viscosity of the oil is measured and given a number, which some people also refer to as the "weight" (thickness) of the oil. The lower the viscosity rating or weight, the thinner the oil. The higher the viscosity rating, the thicker the oil. Viscosity ratings for commonly used motor oils typically range from 0 up to 50. A "W" after the number stands for "Winter" grade oil, and represents the oil's viscosity at zero degrees F. Low viscosity motor oils that pour easily at low temperatures typically have a "0W", "5W" or "10W" rating. There are also 15W and 20W grade motor oils. Higher viscosity motor oils that are thicker and better suited for high temperature operation typically have an SAE 30, 40 or even 50 grade rating. These numbers, by the way, are for "single" or "straight" weight oils. Such oils are no longer used in late model automotive engines but may be required for use in some vintage and antique engines. Straight SAE 30 oil is often specified for small air-cooled engines in lawnmowers, garden tractors, portable generators and gas-powered chain saws. Engine Lubrication System MULTI-VISCOSITY OILS Most modern motor oils are formulated from various grades of oil including base stocks refined from crude oil and recycled re-refined oil. The base stock determines the lubrication characteristics of the oil. Multi-viscosity oils contain polymer "viscosity index improvers" that alter the way the oil flows at both high and low temperatures. Multi-viscosity oils flow well at low temperature for easier starting yet retain enough thickness and film strength at high temperature to provide adequate film strength and lubrication. A thin oil such as a straight 10W or even a 20W oil designed for cold weather use would probably not provide adequate lubrication for hot weather, high speed driving. Likewise, a thicker high temperature oil such as SAE 30 or 40 would probably become so stiff at sub-zero temperatures the engine might not crank fast enough to start. Multi-viscosity grade oils have a wide viscosity range which is indicated by a two-number rating. Popular multi-viscosity grades today include 0W-20, 5W-20, 5W-30, 10W-30, 10W-40 and 20W-50. The first number with the "W" refers to the oil's cold temperature viscosity, while the second number refers to its high temperature viscosity. WHICH OIL VISCOSITY TO USE? Most vehicle manufacturers today specify 5W-20 or 5W-30 motor oil for year-round driving. Some also specify 10W-30 or 0W-20. Always refer to the vehicle owners manual for specific oil viscosity recommendations, or markings on the oil filler cap or dipstick. As a rule, overhead cam (OHC) engines typically require thinner oils such as 5W-30 or 5W-20 to speed lubrication of the overhead cam(s) and valve-train when the engine is first started. Pushrod engines, by comparison, can use either 5W-30, 10W-30 or 10W-40. As chilometroage adds up and internal engine wear increases bearing clearances, it may be wise to switch to a slightly higher viscosity rating to prolong engine life, reduce noise and oil consumption. For example, if an engine originally factory-filled with 5W-30 now has 100,000 chilometri on it, switching to a 10W-30 oil may provide better lubrication and protection. For sustained high temperature, high load operation, an even heavier oil may be used in some situations. Some racing engines use 20W-50, but this would only be recommended for an engine with increased bearing clearances. Increasing the viscosity of the oil also increases drag and friction, which can sap horsepower from the crankshaft. That's why 20W-50 racing oil would not be the best choice for everyday driving or cold weather operation for most vehicles. The latest trend in racing is to run tighter bearing clearances and use thinner oils such as 5W-20 or even 0W-20 to reduce friction and drag. ADDITIVES MAKE THE OIL Up to 25% of the liquid in a typical quart of oil is additive. Additives are what really make the oil and determine its performance properties. Additives extend the viscosity range of the oil, allow it to withstand high pressures and loads, handle contaminants in the crankcase, and reduce friction for improved fuel economy. One of the most important additives is "Viscosity Index (VI) Improvers". These help the oil maintain a consistent viscosity as temperature and load change. "Pour point depressants" are also used to prevent the oil from thickening at low temperature for easier starting. Modern motor oils also contain detergents and dispersants to reduce varnish and sludge formation to keep the engine clean. There are also "anti-oxidants" to minimize oil burning when the oil gets hot. This also helps reduce the formation of varnish and carbon deposits inside the engine. Rust and corrosion inhibitors are added to counteract the harmful effects of water, unburned fuel and exhaust gases that blow past the rings and enter the crankcase. This prevents the formation of acids that can pit bearing surfaces. "Foam inhibitors" are used to minimize the formation of air bubbles as the oil is churned by moving parts. "Wetting agents" help the oil stick to hot surfaces so it doesn't run off and leave the metal unlubricated and unprotected. Finally, there are "anti-wear" and "extreme-pressure" additives. These include zinc and phosphorus (ZDDP) that provide wear protection when metal touches metal. A small amoun tof molybdenum may also be used as an anti-wear additive. Racing oils typically have a higher dose of ZDDP to provide extra protection in high revving, high load applications. Older pushrod engines with flat tappet camshafts also requires higher levels of ZDDP in the oil. For more on this subject, see ZDDP – What is it and Why do you need it?. API MOTOR OIL SERVICE CLASSIFICATIONS The "service rating" of motor oils is classified by the American Petroleum Institute (API). The program certifies that an oil meets certain OEM quality and performance standards. The service rating is shown in the API "Service Symbol Donut" on the product label. There may also be an "API Certified for Gasoline Engines" seal on the label. API Gasoline Engine Oil Specifications The latest service category rating for gasoline engines is SP. Introduced in May 2020, motor oils that meet the SP rating are designed to provide protection against low-speed pre-ignition (LSPI), timing chain wear protection, improved high temperature deposit protection for pistons and turbochargers, and more stringent sludge and varnish control. API SP with Resource Conserving matches ILSAC GF-6A by combining API SP performance with improved fuel economy, emission control system protection and protection of engines operating on ethanol-containing fuels up to E85. The previous API Service rating was SN for 2011 to 2020 model year vehicles (except 2020 models that require SP rated oils). This rating replaced the previous SM rating that was introduced in November 2004 for 2005 and newer engines. SN-rated oils along with the previous SM, SL and SJ ratings, are all backwards compatible and can be safely used in older engines. But the opposite is not true. Older obsolete service classifications (SH, SG, SF, etc.) may not meet OEM lubrication requirements for newer engines. Likewise, API SL oils should not be used in 2005 and later vehicles, and SJ oils should not be used in 2001 and newer vehicles. API also gives oils an "Energy Conserving" rating if the oil meets certain criteria for reducing friction and oil consumption, and improving fuel economy. Most 5W-20 and 5W-30 oils meet this classification. Click here for information about API Motor Oil Specifications for 2020. GF-Rated Motor Oils The International Lubricant Standardization and Approval Committee (ILSAC) is similar to API except that it establishes ratings for motor oils in European and Asian vehicle applications. GF-6A oils were introduced in May 2020 to provide protection against low-speed pre-ignition (LSPI), timing chain wear protection, improved high temperature deposit protection for pistons and turbochargers, more stringent sludge and varnish control, improved fuel economy, enhanced emission control system protection and protection of engines operating on ethanol-containing fuels up to E85. The GF-6A rating is similar to the API SP rating. GF-6B are a special class of low viscosity oils with an SAE viscosity grade of 0W-16. Introduced in May 2020, GF-6B oils provide protection against low-speed pre-ignition (LSPI), timing chain wear protection, high temperature deposit protection for pistons and turbochargers, stringent sludge and varnish control, improved fuel economy, emission control system protection and protection of engines operating on ethanol-containing fuels up to E85. In October 2010, the ILSAC introduced the GF-5 rating that provided better oil life, fewer deposits, less sludge and improved protection for turbochargers and the catalytic converter. GM Oil Specificati…

Fonte: AA1Car.com