Perhaps I misstated. Multi weights oils, as I understand, when they get hot
only thin out to the point that their high weight number would at the same
temp.
Something about polymers uncoiling into long chains.

In other words think of a 15-40W oil as a 15W oil that will not thin more
than
a 40W would when hot.

Like I said, that's my understanding. But, I could be wrong. That's why I
asked
the question. :-)

Capt. Bill

That is correct.