Read the Ford Dealer Service Bulletins for info on thinning the 600W oil with kerosene for winter.
There is not a "modern SAE 680" oil. The 680 is on the ISO scale. It is a different lab test from the SAE test. See the table below.
"600W oil" was a Mobil Oil trade name for a specific oil they sold, it is NOT 600 WEIGHT oil. Sorry, guys; common misunderstanding. It was called Mobil 600W Steam Cylinder Oil. It was thick, it was sticky. It clings to the flat gear faces of our transmission and differential quite well, which provides an oil cushion between the metal surfaces to prevent wear. It also slows the spinning gears inside the transmission when you shift, eliminating the need for double-clutching for most up-shifts (1st to 2nd, 2nd to 3rd) unless you are drag racing.
IF the original 600W oil was indeed the ISO 680 available today, and I have no documentation to say that it is, then that is equivalent to SAE 190 Gear Oil. There is also a modern version of 600W that is ISO 460. This is close to SAE 140. I need to go to the Benson Ford Research Center to ty to find out what the original ford specification was for this oil.
The figure also helps you equate the ISO 1200 to SAE 250. This chart is very helpful, I refer to one like it a lot.
A multi-viscosity oil might make sense in a cold climate. Remember the numbering system of SAE 75W-140 means it acts like SAE 75 oil in Winter (the W) and like SAE 140 oil in the summer. So that might work well in Alaska. Or 85W-140. Something like that. Remember, Ford did not have these fancy multi-grade oils in 1928-31.
Hope this helps.