Viscosity is considered the most important single property of lubricating oils. Automotive crankcase and gear oils used in gasoline and diesel engine lubrication have been classified by the SAE (Society of Automotive Engineers, USA) in terms of viscosity. The viscosity classification takes into account the temperature at which the oils are to be used. In 1911, the SAE developed the SAE J300 standard that specifies engine oil viscosity classification. The SAE viscosity rating was based on average time, in seconds, for a specific quantity of motor oil at 212°F to flow through a test apparatus or viscometer with an orifice of a specified diameter. An SAE viscosity number, according to SAE J300, was the average time and not an exact measure. For example, any oil that took 5 to 15 s to flow would be rated as SAE 10. Similarly, oil that took 25 to 34 s would be SAE 30, and so on. At that time, only SAE 10, 20, 30, 40, and 50 grades existed. There were no SAE 0 or SAE 60 or winter grades as in the present classification. Over the years SAE J300 standards were amended many times with advances in automobile design. SAE grade 60 was added as the need for thicker oil in aviation and heavy-duty engines became apparent. SAE W grades were added as it became apparent that engines could not be started in cold winter conditions with some SAE 30 oils. Table 11-2 shows the SAE classification for crankcase oils.

A multigrade engine oil such as SAE 10W/30 has the viscosity characteristics of both SAE 10W and SAE 30 oil. Similarly, the multigrade grade oil SAE 80W/90 has the viscosity characteristics of SAE 80W and SAE 90 gear oils. Today, most automobiles and trucks use what is termed “multiviscosity oils.” These are in fact multigrade, not multiviscosity, oils. An oil cannot be multiviscosity, but it can be multigrade. For example, an SAE 10W40 oil meets the viscosity requirements of SAE 40 oil and also that of subzero grade SAE 10W. This implies that oil will flow like SAE 10W oil at a low ambient temperature such as during cold starting. Referring to Table 11-2, an SAE 10W oil will have a viscosity not greater than 7000 cSt at –25°C (for cold cranking) and not more than 60,000 cSt at –30°C
to facilitate the flow of oil to the engine during cold weather. Multigrade lubricating oils are manufactured by blending a low-viscosity base stock oil with a viscosity index improver additive. These are polymer additives that produce a thickening effect at high temperatures but are dormant at low temperatures. In multigrade oils, polymers are added to light base stock that prevent the oil from thinning out at a fast rate as the oil warms up. At low temperatures, the polymer molecules are coiled up and allow the oil to flow without freezing. As the oil warms up, the polymer molecules begin to unwind into a long chain that prevents the oil from thinning at a rapid rate with the temperature increase. Multigrade oils are one of the greatest advancements in lube formulation, but they should be chosen with caution. It is appropriate to choose a multigrade oil with the narrowest span of viscosity appropriate for the highest and lowest ambient temperature in summer and winter driving conditions. Lubricating oil with a minimum amount of polymers is better for the engine. In actual use in engines, polymers can shear and burn, forming deposits that can cause sticking and other problems in engines. For example, if we compare the two lube oils 10W-40 and 20W-50, both oils have a 30-point spread, but because 20W-50 starts with a heavier base oil, it requires less of a viscosity index improver (polymer) to meet the specifications and thus is preferable. The wide viscosity range oils with their higher polymer content are more prone to viscosity and thermal breakdown. It is the oil that lubricates, and additives do not play any part in it. The oils with the minimum of additives are the best. The optimum viscosity grade for a given application depends on the ambient conditions and the type of engine used.

In certain applications, multigrade engine oils with a high VI additive content is not desirable. These include high r/min, high-load engines such as lawn mowers, high horsepower racing cars, and diesel engines. Lubricants such as SAE 30 or SAE 40 are recommended because these engines operate under a very high r/min, load, and heat conditions and, in the case of small engines, they are air cooled. VI additives are a weak link in the lube. VI additives under extreme heat, load, and shear condition tend to depolymerize, resulting in failure to reach their intended high-temperature viscosity. As the shearing continues, loss of oil film, increased wear rates, high oil consumption, or engine failure may result. Use of synthetic (PAO, esters) base oil lubes with very high VI (120+) is increasing in this segment.

The ideal viscosity for motor oil in piston engines operating at normal engine operating conditions is equivalent to SAE 30 (9 to –12 cSt at 100°C). If a thinner oil is used, say SAE 20, there will be less resistance to motion due to reduced viscosity and therefore better fuel economy. However, this gain in fuel economy does not occur without cost due to the following factors:
• Lower viscosity and higher volatility results in greater oil consumption.
• Decrease in engine service life due to increased engine wear under the same operating conditions.
It is estimated that the gain in term of better mileage by using SAE-20 instead of SAE-30 may be less than 1 percent, whereas the reduction in engine life may be on the order of 30 percent. If a heavier oil (SAE 40 or SAE 50) is used under normal operating conditions, there will be more resistance to motion due to increased viscosity, resulting in poorer fuel economy. The loss in fuel economy is somewhat compensated for by the following:
• Decrease in oil consumption due to higher viscosity
• Decrease in oil consumption due to lower volatility
• Increase in engine service life due to reduced engine wear
Most heavy-duty engine manufacturers recommend SAE-40, 15W-40, or 5W-40 oils. If the ambient or the operating temperature is increased from ideal or normal (70°F), oil viscosity must be increased to assure same level of protection from engine wear, thus SAE 40 for 100°F and SAE 50 oils for 120°F ambient temperatures. Heavier grades are also desirable for towing and heavy loads. If the engine speed is increased during long-distance high-speed driving in low ambient temperatures, lower viscosity oils such as SAE-20 could be used in place of SAE-30 (in manual transmission cars where vehicle speed is proportional to engine r/min). If the load is decreased, oil viscosity can be decreased without any penalty. Long-term durability of the engine is related to high temperature/
high shear specifications in SAE J300 standards. For SAE-20 it is 2.6 cP minimum at 150°C. For SAE-30, it is 2.9 cP (minimum). For SAE-40 oil, there are two specifications: 2.9 cP, which is the same as that for SAE-30, and 3.7 cP, which is the same as those for SAE-50 and SAE-60. The first specification is for light-duty engines (service life, 100,000 to 150,000 miles), and the second specification is for heavy-duty engines (service life, up to 1,000,000 miles). These oils are labeled as “HD” oils. Heavy-duty oils must meet the second SAE-40 specification of 3.7 cP, minimum, at 150°C. (Refer to Table 11-3 for a list of the optimum motor oil grades.)