I think Todd was referring to the effect temperature has on oil viscosity, and hence on pressure. Hotter oil will be less viscous and better able to flow through pipes, galleries and bearing clearances, so one would expect lower OP at the same rpm with hot straight SAE oil than cold. Which is one reason why I have always used multigrade oils.
I'm old enough to remember when no such products existed. You used SAE30 or 40 (depending on the age of your engine) in the summer, and changed to SAE20 in the winter. I remember the changeover ritual - rather like replacing your storm windows with bug screen, and the reverse. My Triumph motorbike was different - it used straight SAE50 year round. Air cooled engines are different.
I know, I know. There are a number of people who will insist that multigrade oil will leak out of our engines. That makes no sense logically. At the temperature the "40" in a 10W40 oil refers to, the oil will be no "thinner" than straight 40. And if you decide to boost the OPRV spring rate, a multigrade will certainly help to avoid overpressure trouble in cold weather.
(Anyone using "weight" should have the fleas of a thousand camels infest their armpits, to quote Johnny Carson.)