If you hopped in Doc Brown’s DeLorean and went back to 1965, your dilemma would make sense. People would look at this multigrade discussion and say “impossible!”…because it was.
NOW, due to the beauty of human ingenuity…it is possible. Think back to my Buick. 10W and 10% kerosene. Kerosene. So once that thing warms up, and maybe even the owner dons a blanket over the radiator or maybe it’s a Duesenberg not a Buick so it has fancy thermo-shutters on the front to cover the radiator, thermostat helps out and it still reaches 160-180F despite being at say Nome Alaska in the winter, how well is that watery oil going to do? Terrible.
One of the many reasons 100,000 miles or so and an engine was spent back in the day, hell people considered the whole car to just be junk for the hell of it. To run that 1930-something right without wearing its bearings out, you should TECHNICALLY…yup…change the oil and dump in a thicker weight once it has warmed up, way up there in Nome. Silly. Now, multigrade essentially does just that. Now being 1980 or 90-whatever when it first came out lol.