Quote:
Originally Posted by TimMullen
Not according to my understanding. The "W" is commonly considered to mean "Winter", but it just specifies that it's a multi-viscosity oil.
Again not my understanding. They start with the lower viscosity oil, and add additives to make it equivalent to the thicker viscosity when hot. As the oil breaks down, the oil viscosity tends to fall somewhere in between the two ratings - it acts like a thinner oil... 
But I could be wrong... |
W stands for winter. SAE 20W is a straight vis oil. SAE 20 is the same vis at 100C, but has no cold vis requirements.
Right, we start with a lower vis oil and add additives (polymers). This comment about base oil thickening was in response to an earlier incorrect post that said Group III base oils shear. Polymers can break down (shear), thinning the oil. But the base oil, the lower vis oil we started with, thickens as it breaks down (oxidizes). So, you are correct, and a broken down 5W-40 could potentially become a 10W-30.