Interesting story. Our '50 Pontiac (straight 8) called for straight 10 weight oil in winter, unless it was Alaska level way sub-zero cold, then either 5 weight or 10 weight cut with kerosene. We used 30 in summer, 20 in mid seasons, and 10 in winter, changing 4x/year. We also used thinner manual transmission and differential fluids in winter and even then the car wouldn’t shift on zero degree days until driven 3-4 miles. When 10W30 was introduced it worked fine for cold starts but oil consumption went up - I later worked with a petroleum chemist who explained that the viscosity improving molecules employed then sheared down quickly, we found this to be true even with an '82 Dodge.
The Pontiac would barely crank on the coldest days and we resorted to bringing the battery inside and and immersing it in a tub of warm water. Lots of people used oil heaters, the best was a friend’s that circulated warm coolant.
Simple answer, since 6V is practically unavalable, switch to 12 volt and if you have anything incompatible use a resistor when necesary.
As far as positive vs. negative ground goes, the switch occured during the early 1950’s but regardless, the polarity had no efffect on the engine operation.
My co-worker buddy drove an older car with positive ground. Sort of a seldom-seen sports car, UK manufacturer I think. Engineer, he enjoyed to experiment with stuff. He decided to install a negative ground radio in his car. Car was expecting radio chassis to be positive, so he had to isolate the radio chassis from the car chassis. Wasn’t entirely successful, b/c after that the car would stall pretty every time he pushed the buttons to switch stations… lol …
A 6-volt cable actually has to have four times the cross-sectional area and thus copper weight in order to match the transmission efficiency of a 12-volt cable.
For example, let’s assume 1/100 of an ohm cable resistance while delivering 200 amps to a 12-volt starter. The 12 volts at the battery becomes 10 volts at the starter delivering 83% of the power to the starter. In order to deliver 83% of the power from a six-volt battery to the starter, the voltage drop can only be 1 volt, 5 volts to the starter. But the 6-volt starter has to draw 400 amps to do the same work. In order to drop one volt at 400 amps, the cable resistance would have to be 1/400 an ohm.