Originally posted by IslandCobra
It is a 5 weight oil that has thickers added. These thickeners make it thin out at a slower rate than if it was a straight 5 weight oil. In turn this means it behaves like a 30 weight oil at temperature. The oil does thin with temperature, it just doesn't thin as much.
Right. Which means that for any given "cold" weight such as 5 or 10, there is (at least theoretically) some very high viscosity that thins to this viscosity at high temperature, so that oil, when warm, has the same viscosity as this oil, when cold. That is what I was referring to as the "crossover point".
But it turns out that 60 weight oil is never thicker than 26.1 cSt at high temperature, and 0 or 5 weight oil is never thinner than 50 cSt at room temperature, so the "crossover point" for a 0 or 5 weight oil would be higher than 60 weight oil. And the SAE doesn't go higher than 60 weight for engine oil (it does, but only for gear oil).
If the crossover point were reached, then we would have a "perfect" oil that does not thin with temperature. But this does not exist. Even a 0W60 oil (if such an oil is chemically possible) would lose half its viscosity at high temperature, from about 50 cSt to about 26 cSt.
That's what I was looking for: how much do these oils actually thin out from cold to engine operating temperature. Here are the rough numbers:
A 5w30 oil loses approximately 80-85% of its viscosity.
A 10w40 oil loses approximately 75-80% of its viscosity.
That's when new and fresh. They lose more as they get old and start breaking down.
It would be a LOT
more logical and intuitive if they just relabeled a 10w40 oil as 65-15. This would represent its ACTUAL
viscosity when cold and warm.