... The coolant slowly circulates by gravity and the entire engine will eventually warm up if given some time.
I think you mean convection? I had a Datsun 240Z many many years ago that I bought in the summer. When winter came I could not start the car because it was turning over very slowly. Realized the previous owner was running 20w-50 and it was like molasse in winter. The car had a lower radiator hose heater, so I plugged it in, waited a couple hours and got the car started. Then, I change the oil to 10w-30 and never used the heater again.
