Bob, I'm showing my ignorance here...but how could that be? You're saying a 15W50 has a viscosity of a 15W
oil when cold, and the viscosity of a 50W
oil when hot? Wouldn't that be a lot thicker once hot? I can't imagine a
oil getting thicker and thicker as heat increases.
I thought the relation between the numbers is just that....the second number is not a "weight" figure, but just a relation of how much it does thin once hot. As stated earlier, a 10W40 would not be as thin at temp as a 10W30 oil would be.
Once again, I could very much be wrong...