Originally Posted by onehundred80
Oil gets thinner as it gets hotter. Full stop.
Multi-grade oil is selected on the basis of initial starting temperature but at normal operating temperature it has to stay thick enough so it isn't useless. At low temperatures it has to be thin enough to work until the engine reaches the normal operating temperature.
If you lived in a climate with a constant temperature you would not need a multi grade oil. The reason being that the correct oil would be thin enough in the cold engine and remain thick enough at the operating temperature.
In a warmer climate you could use a 5W or a 10W oil which would be damaging to do in a cold climate.
The oil film has to be of a sufficient thickness to prevent metal to metal contact. The oil forms a cushion on which parts move, if the cushion is too soft (oil to thin) it will get squeezed away, to hard (oil too thick) and the cushion cannot form.
That's the way I see it.
The Canadian Crossfires at least (maybe all Crossfires) come with a block heater and this would have to be used when the 0W-40 oil was not suitable for the ambient temperature. The heaters use would be beneficial on cold days anyway.
Multi-grade oil really means that the chemistry is such that the oil maintains a more uniform viscosity index across a wider temperature range. For single weight oils, the hotter it gets, the 'thinner' it gets. Multi-grade oils perform equally as well at both low & high temperature ranges.
When a car engine is cold, you really want an oil that is 'thin enough' to rapidly move thru the engine during start-up to avoid significant wear. Maybe the single worst time for engine wear. And the oil then needs to remain 'thick enough' at operating temperatures to provide the same protection. The multi-grade oils of today can do this better than ever and especially the synthetic grades. Nearly all of the OEM's producing high performance vehicles will recommend a synthetic oil for the engines for this very reason.
And
180 is correct when he states that for oil to be successful in its performance it has to be able to support a thin film under pressure between metal parts (hydrodynamic lubrication with constant oil supply pressure).