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[bobcowan]

An oil molecule is the same size and weight, always. It never changes. No matter what the brand or viscosity, the oil molecule is the same size and weight. That's a constant. Just like any other molecule. To change the size/weight, you need to add or subtract atoms. When you do that, you create a new substance. That's basic chemistry.

Nowhere in that article does it mention molecular weight.

SAE grade's the oil rating on it's viscosity at a specific temperature. That's it. that's all it's based on - viscosity. The ratings change a bit through the years as oils and engines advance. But it's still based on viscosity.

The manufacturer's change the viscosity with additives, as said above. That doesn't change the basic molecule, but does change the solution. Like making kool-aid.