When you look at the viscosity on the oil bottle, the first number be it 0W, 5W, 10W, etc… is the WINTER weight.
This means the oil will flow at that viscosity down to zero degrees. Below that, the oil starts to thicken. Full synthetic oil doesn’t start to thicken until it reaches -40-50 degrees.
But at that temperature, nothing moves anyway.
I’ve maintained my 97 Accord since it went out of warranty with original owner. And the engine has seen nothing but 5W-30 regular oil.
It’s now got 230,000 miles on it and running strong.
Synthetic oil? Application specific.
Tester