1 winter 100 years ago, I had to change the oil in a VW beetle, and all we had was straight 30. It didn't crank slowly: even with a jumper to add a 2nd battery the starter didn't turn the flywheel at all. We had to tow the car to start it. Gene Berg said more Type I engines blew up from using heavy oil than any other detectable cause.
At cruising speed, heavier oil has already opened the relief valve, so the local pressure at the journal face has not improved.
My joke: in a multigrade oil, the number before the "W" is the time in seconds for oil pressure to reach the farthest bearing, tappet, rocker arm.
20W50: 20 seconds
5W40 (which I use in my 2JZ engine): 5 seconds
