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When it comes to fleet and generator maintenance, nothing has a bigger impact on engine-life cycle than a simple oil and filter change. Yet, as straightforward as an oil change sounds, effectiveness is only assured by using the correct lubricant viscosity. If you own, operate, or maintain a vehicle, you are likely aware that engine oils are designated by their oil base (synthetic versus non-synthetic), and their viscosity grade. Almost all automotive oils in use today are multi-grade products that are available in viscosity ratings ranging from 0W5 to 20W50.

Choosing the correct viscosity is critical for the engine life cycle. A lubricant’s primary function is to reduce and control friction and wear, by providing a load carrying oil film strong enough to sustainably separate two surfaces moving over one another. The load-carrying ability of any lubricant is determined by its viscosity rating.


Viscosity, arguably oil’s most important property, signifies a lubricant’s measure of resistance to flow, defined by molecule size. The larger the molecule size, the thicker or more flow resistant the oil will be. Low-viscosity oils flow faster because their smaller molecules slide past and over each other more easily. Viscosity is a unique property in that it thickens or thins according to the lubricant’s temperature. As oil gets colder it will thicken, and inversely, will thin out when gets hotter.

Up until 1952 (more about the importance of that year later), all vehicle manufacturers recommended mono-grade (single-viscosity grade) automotive crankcase lubricants for their vehicles. This meant that recommended oil for use in summer months would need to be changed out to a less viscous (lower-grade number), single-grade oil before the winter driving season.

The correct viscosity grade for an automotive-engine crankcase has typically depended upon where you live and operate your vehicle. Due to cold winters, northern-climate vehicles subject engine and gearbox oils to very cold temperatures on startup. Cold, viscous oil results in poor (sluggish) oil flow, and a momentary lack of full film lubrication until the oil reaches its operating temperature. Consequently, this lack of flowing oil and drag on the engine translates into excessive engine wear during the start up/warm up stage.

Originally, engine-oil mono-viscosity grades were set by the SAE (Society of Automotive Engineers). The SAE oil grade viscosity system designated winter use oils (low viscosity) with a number followed by the letter “W,” e.g., 10W, 15W, 20W. That “W” simply meant that the product was a “Winter” oil. Similarly, summer-use oils were designated with a number, but no letter, e.g., 20, 30, 40, or 50. However, the numbering system changed significantly in 1952, when Esso (now Exxon-Mobil) introduced its Esso “Uniflo” product: the world’s first multi-grade oil.

Multi-grades offered motorists the advantage of a single oil selection for year-round driving that delivered improved low-temperature startups and high-temperature performance. These multi-grade products were designed to provide a stable oil viscosity thickness throughout multiple defined temperature ranges. Each viscosity number reflects a different temperature-operating range from which a consumer could make a suitable match choice based on geographical location and temperature.

The multi-grade viscosity number is determined by first identifying the base oil viscosity for cold weather performance. This is followed by the viscosity grade the oil will perform at (or emulate), once it reaches its operating temperature. For example, a 20W50 has relatively thin SAE20W winter base oil that will “thicken up” and act similar to more viscous SAE50 oil at operating temperature to provide full film protection over a wider temperature range. This is achieved by blending polymeric viscosity improver additives to low-viscosity base oil. These additives are long-string polymers that “curl up” like a ball at low temperatures and move freely among the oil molecules. Once the oil heats up, the polymer strings unfurl and expand to restrict the oil’s flow and raise its apparent viscosity. Whether the base oil is a conventional mineral oil or synthetic, the viscosity multi-grading system remains the same.


Over the past 50 years, multi-grade lubricants have improved significantly and all but replaced mono-grade products, which are still used for small summer-use equipment, i.e., lawn mowers, among others. Viscosity choice, though, is still dependent on operating temperature conditions. Moreover, the cardinal rule still exists when deciding on the correct viscosity oil: Read the vehicle operator’s manual and select the viscosity range best suited to your main operational climate.TRR


Ken Bannister has 40+ years of experience in the RAM industry. For the past 30, he’s been a Managing Partner and Principal Asset Management Consultant with Engtech industries Inc., where he has specialized in helping clients implement best-practice asset-management programs worldwide. A founding member and past director of the Plant Engineering and Maintenance Association of Canada, he is the author of several books, including three on lubrication, one on predictive maintenance, and one on energy reduction strategies, and is currently writing one on planning and scheduling. Contact him directly at 519-469-9173 or

Tags: reliability, availability, maintenance, RAM, lubrication, lubricants, oil, viscosity, filtration, oil, Exxon-Mobil