Motor Oils: Decisions

When deciding between motor oils, picking one with proper viscosity is crucial.  What is viscosity?  It is an oil's relative resistance to flow at differing temperatures.  Phrased differently, an oil must be "thin" enough to flow at low temperatures and "thick" enough to provide engine protection at high temperatures.  Thus, an ideal oil will maintain its viscosity over a wide temperature range.  This, in fact, is an oil's viscosity index: the measure of its propensity to change viscosity with temperature changes.  The higher an oil's viscosity index, the more consistent its viscosity over a range of temperature changes.

How does all of this impact how we choose motor oil?  Naturally, it is best to choose an oil with a viscosity index best suited to your particular needs.   Monograde oils, such as an SAE 30 oil, are designed for use under consistent temperatures.  These are the oils often used in lawn mowers, as it is assumed that mowers will be used almost exclusively during the summer months.  They have a low viscosity index and will thicken quite a bit if used under cool conditions. Multi-viscosity motor oils, on the other hand, are designed for
use where temperatures vary greatly.  An example would be a 5w30 oil.  The first number indicates how easily it will allow the engine to turn over at low temperatures (the lower the number, the better the cold weather operation).  This number is determined through the use of two tests: the Cold Crank Simulator Test and the Mini-Rotary Viscometer test.  The better the startability at low temperatures, the lower the "w" classification of the oil.  As you can see, multi-viscosity oils are the better oils for most passenger car
automobiles, especially those used in climates with wide temperature ranges.

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