[skyship007]

Originally Posted by canis

Right, let's clear some things up here:

In oil viscosity ratings, the first number is it's performance when at operating temperatures. So saying a 0W oil helps cols starts is nonsense. The second number is it's performance when cold, so saying a 30W oil is thinner than a 40W oil is only stating the obvious.

While thicker oil does necessitate extra drag, it is miniscule compared to the effort required to turn two camshafts. Next time you've got the belt off, try it. The starter motor extra drag when cold is caused by metal contraction, but even that doesn't really compare to the effort to overcome valve springs. The starter motor is unaffected by oil viscosity. This extra strain 40W oil puts on a starter motor is imaginery, as my 188,000m motor has verified today.

You have got it all backwards, the first number relates to the viscosity when cold, so in artic conditions you should use an 0W oil.
The W is for winter, as it relates to the viscosity at minus 18C, the second rating is for plus 100C.

There is a real difference in the load on the starter and battery when cold starting an engine when using an 0W rather than a 10W, although it would need to be a real cold day for that difference to be audible.

The difference in fuel economy only relates to the period the engine is warming up in terms of the W figure, once its warm the second rating is of more importance, although the differences are not very significant. The additives in the oil also have a slight effect, so some of the modern oils have more Moly (Mo) additives than required in wear protection terms, as it reduces friction slightly.