Winter starts: does the W number really matter?
Below freezing, the difference between a 0W and a 10W oil is measured in seconds of unprotected running.
What happens at start-up
For a moment after a cold start, parts are turning with only residual oil on them. The thinner the oil at that temperature, the faster the pump can deliver a full film. This is where the majority of measurable engine wear occurs, and it is what the W rating exists to describe.
Why 0W is now common
Modern engines have narrower galleries and tighter clearances, and increasingly rely on oil pressure to operate timing components. A 0W oil reaches operating pressure faster in cold weather, which is why manufacturers have moved that way across almost every range.
Older cars in a cold snap
If your handbook offers a range, use the thinner end of it in winter. A car specified for 10W-40 or 5W-40 will usually accept 5W-40 year round, and the cold-start benefit is real.
What it will not fix
A slow crank in the cold is a battery problem, not an oil problem. Thinner oil reduces drag slightly, but if the starter is labouring the charging system is where to look.
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