Monograde and 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.

Automotive Lubricant Viscosity Grades Engine Oils – SAE J 300
SAE	Low Temperature Viscosities		High-Temperature Viscosities
Viscosity Grade	Cranking (mPa/s) max at temp °C	Pumping (mPa/s) max at temp °C	Kinematic (mm2/s) at 100°C		High Shear Rate (mPa/s) at 150°C
			min	max	min
0W	6200 at -35	60 000 at -40	3.8	—	—
5W	6600 at -30	60 000 at -35	3.8	—	—
10W	7000 at -25	60 000 at -30	4.1	—	—
15W	7000 at -20	60 000 at -25	5.6	—	—
20W	9500 at -15	60 000 at -20	5.6	—	—
25W	13 000 at -10	60 000 at -15	9.3	—	—
20	—	—	5.6	<9.3	2.6
30	—	—	9.3	<12.5	2.9
40	—	—	12.5	<16.3	2.9
40	—	—	12.5	<16.3	3.7
50	—	—	16.3	<21.9	3.7
60	—	—	21.9	<26.1	3.7

Fig.1 SAE J300 – Automotive Engine Oil Classifications

 

 

 

Monograde Oils
Monograde oils, as defined by the SAE J300 classification system, cover a single requirement and cannot use a polymeric viscosity index improver (also referred to as a viscosity modifier).  SAE J300 has established eleven viscosity grades see fig.1, of which six are considered Winter-grades and given a “W” designation. For single winter grade oils, the dynamic viscosity is measured at different cold temperatures, specified in J300 depending on the viscosity grade, in units of mPa/s. Based on the coldest temperature the oil passes at, that oil is graded as SAE viscosity grade 0W, 5W, 10W, 15W, 20W, or 25W. The lower the viscosity grade, the lower the temperature the oil can pass. For example, if an oil passes at the specifications for 10W and 5W, but fails for 0W, then that oil must be labeled as an SAE 5W. It cannot be labeled as either 0W or 10W.
For non-winter grade oils, the kinematic viscosity is measured at a temperature of 100 °C  in units of mm²/s or the equivalent centistokes. Based on the range of viscosity the oil falls in at that temperature, the oil is graded as SAE viscosity grade 20, 30, 40, 50, or 60. In addition, for SAE grades 20, 30, and 40, a minimum viscosity measured at 150 °C and at a high-shear rate is also required. The higher the viscosity, the higher the SAE grade is.   
Remember that for any oil, if temperature is increased, viscosity will decrease. See Fig. 2

Fig.2 Viscosity/Temperature Graph
The viscosity is high at low temperatures and low at high temperatures.
However, not all oils behave in the same manner. Some oils thin out less than others when the temperature is increased. This is the difference between a monograde and a multigrade oil.

 

Multigrade Oils
As opposed to monograde oils which cover a single requirement as stipulated by the SAE J300 classification system, multigrade oils meet the requirements of more than one SAE grade and may therefore be suitable for use over a wider temperature range than a monograde oil.  Multigrade oils are made by blending a low-viscosity oil with special additives called Viscosity Index Improver (VII) VII are additives which improve the temperature/viscosity characteristics of oils. Oils containing these additives, combine the good starting and friction properties of a thin oil at low temperatures with the good lubricating properties of a more viscous oil at high temperatures. See  Fig. 3

Fig. 3   Viscosity Index Improvers

 

 

 

The mechanism by which these polymers work, is that at low temperature, they curl themselves up into a tight ball that moves freely with the oil molecules. As the temperature increases though, they expand into a large stringy structure which restricts the normal flow of the oil. As the temperature reduces, they go back to their original shape. The result is that for example, when these polymer additives are blended in the correct proportion with an SAE15W oil, the oil flows like an SAE 15W oil at low temperatures and like an SAE 40 oil at high temperatures. The result is an SAE 15W/40 oil that will provide wide protection over an extended temperature range. See Fig. 4

Fig. 4 Viscosity/Temperature Characteristics of Monograde & Multigrade Oils

Advantages of Multigrade Oils

• One oil for year-round use
• Improved low-temperature starting
• Excellent high-temperature performance
• Improved overall fuel economy -  less idling time and faster warm up
• Less battery drain on cold starts
• Fast
• er, full-pressure lubrication over a wider temperature range

Disadvantages of Multigrade Oils:

• Low temperature oil leaks may occur on badly worn engines
•  Polymer shearing may occur under high stress and/or shock load conditions

Conclusion
Multigrade oils are the preferred option for all modern engines. 
Multigrade oils will keep equipment operating in an optimum viscosity range, and offer consistent response and lubrication protection. 
Monograde oils are an acceptable option if the equipment can provide excellent oil cooling and constant temperature control.