Monograde vs Multigrade Oils

MONOGRADE VS MULTIGRADE ENGINE OILS Introduction An engine oil fulfills a number of crucial functions and failing in any one of them, may be catastrophic. A good engine oil must be able to hold in suspension contaminants such as carbon (soot) particles, by-products of fuel combustion, neutralize organic acids, protect against rust and corrosion and dissipate the heat generated from the combustion cycle. And it must do all of these things under tremendous heat and pressure without succumbing to fatigue. Yet the most important function of a lubricant is to lubricate and reduce friction. This function is performed by the oil film formed between two moving parts and its load carrying ability and is described as the oil’s viscosity. In the common engineering sense, viscosity is the resistance to flow or ―thickness‖ of a liquid. The viscosity decreases with an increase in temperature, hence the temperature at which a viscosity is measured, must always be specified. There are several viscosity units but the one most frequently used is the Centistoke (cSt), which is the derived unit of kinematic viscosity where 1 cSt = 1 mm²/s. Crankcase Oil Classifications The most widely used system for engine oil classification is that established by the Society of Automotive Engineers (SAE) in the USA. In this system, two series of viscosity grades are defined – those containing the letter ―W‖ and those without it. Grades with the letter ―W‖ are intended for use at lower temperatures and are based on a maximum low temperature viscosity and a maximum borderline pumping temperature, as well as a minimum viscosity at 100°C. The low temperature viscosity is measured by means of a multi-temperature version of ASTM D2602. This test describes the method for apparent viscosity of motor oils at low temperature using the Cold Cranking Simulator. Viscosities measured by this method have been found to correlate with engine speeds developed during low temperature cranking. Borderline pumping temperature is measured according to ASTM D3829. This provides a measure of an oil’s ability to flow to the oil pump and provide adequate pressure during the initial stages of operation. Oils without the letter ‖W‖, intended for use at higher temperatures, are based on the viscosity at 100°C only. These are measured by ASTM D445. Fig. 1 depicts the SAE J300 classification for engine oils.

SAE Viscosity Grade

0W 5W 10W 15W 20W 25W 20 30 40 40 50 60

Automotive Lubricant Viscosity Grades Engine Oils – SAE J 300 Low Temperature Viscosities High-Temperature Viscosities Cranking Pumping Kinematic (mPa/s) (mPa/s) High Shear Rate (mPa/s) 2 (mm /s) max at temp max at temp at 150°C at 100°C °C °C min max min 6200 at -35 60 000 at -40 3.8 — — 6600 at -30 60 000 at -35 3.8 — — 7000 at -25 60 000 at -30 4.1 — — 7000 at -20 60 000 at -25 5.6 — — 9500 at -15 60 000 at -20 5.6 — — 13 000 at -10 60 000 at -15 9.3 — — — — 5.6