Post #12
spiky wrote:think it's the other way around dude
hence 0-40 is really thin
Nope.
For a multi-weight oil
The first number (0w, 5w, 10w, 15w) etc, is the rating when the oil is cold.
I.E. - It'll be the same thickness as that grade of straight weight oil at that temperature.
The latter number is the equivalent thickness when hot, so the -20,-30,-40,-50 all match up to the same thickness as a straight oil at 100*c.
The main bit
oil is always thicker cold than hot. So even if you had a 0w-60 oil, the oil would still be thicker at the temperatures where the '0w' bit applies than it is at 100*c where the 60 rating does.
In other words, it's impossible to get an oil that is 'too thin' when it's cold.
You choose the latter rating depending on the operating temperature of the engine, you want to be running similar oil thickness as the standard engine when it's running through the bearings, etc, but obviously, if the engine is boosted or highly loaded, there will be more stress and heat there.
So, because the temperatures are higher, the oil will thin out even more, so you might want to go to a 50 weight so that it's the same thickness (from the extra heat) as the 40 weight is under normal conditions.
Obviously, oil quality, bearing clearances and what oil cooling you are running has a big effect on the selection.
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ben306
Posted 9th Jun 2011 at 12:53
