The viscosity rating of a multi-grade oil is broken down into two parts:
The
number before the W is the "winter" or "cold start" rating. The lower
the number the thinner the oil. This allows quick flow when the engine
is started cold (cold crank). 90% of engine wear happens at this time
due to the engine running in a dry state.
The second number is
the viscosity rating of the oil at 100 C. The higher the number the
thicker the oil. If the oil is too thick for the temperatures your
engine is running at, the oil will not circulate freely. If it's too
thin, it may not lubricate effectively.
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