Given this statement, do the polymers adhere to the oil molecules, or just mix in with the oil? On the same line, what happens when you mix a 10w oil with a 90w oil? Polymers are added to a light base (5W, 10W, 20W), which prevent the oil from thinning as much as it warms up. At cold temperatures the polymers are coiled up and allow the oil to flow as their low numbers indicate. As the oil warms up the polymers begin to unwind into long chains that prevent the oil from thinning as much as it normally would.