In the SAE classification, oils are identified according to their
viscosity, not taking account of any other qualitative feature.
The first number refers to cold viscosity for winter use (symbol
W = winter), while the second one considers hot viscosity.
The choice criterion must take account of the minimum environment
temperature the engine will be subjected to for winter and of the
maximum operating temperature for summer.
In the case of sporty and urban driving or when the air temperature
is high, the engine undergoes high temperatures that accentuate
the phenomenon. It is therefore fundamental to use an oil that stays
sufficiently fluid, to distribute well in the engine, protect the
mechanical parts and facilitate ignition.
Monograde oils are generally used when the operating temperature
does not vary much.
A multigrade oil is less sensitive to temperature.
Multigrade oils are a particular class of oils, which have a viscosity
curve that complies with several SAE numbers. For example a 15W-40
oil covers the 15W-20/30/40 grade range.
SAE TABLE OF ENGINE OIL VISCOSITIES (SAE
J300 - Society of Automotive Engineers)
SAE GRADES FOR ENGINE OILS RECOMMENDED
IN RELATION WITH THE OUTSIDE TEMPERATURES