"w" stands for weight
the first number is winter rating lower is better because it will flow better in cold
the second number is operating temperature the higher the rating the thicker it stays and the less it thins out.
As oil heats up, the long chain molecules uncurl from their natural spiral pattern and straighten out, so the oil becomes thinner and flows more readily.
For an example if you heat up syrup it gets thinner but its thicker when cold.
The winter rating shows its suitable for use in the winter.
w50 is able to maintain it's viscosity at higher temperatures as compared to w40.
It has nothing to do with being old and new. API ratings determine the quality of the oil.
