1st a price correction, $12 per quart.
Old Man, thanks for your comments. I have never seen specific weights for diff or trans fluid or grease, except engine oil in Plymouth literature, but I would like to. My Instruction Book is only specific as identifying such as "Fluid Gear Lubricant," Medium Cup Grease." These terms are from The Lubrication Chart in the middle of the Instruction Book, Fifth Edition May 1929. At the time, they were probably very well understood. Interestingly, the Chart specifies SAE numbers for engine oil.
The Chart does say to dilute Rear Axel and Transmission with Kerosene for winter. The 600w may be too viscous, even in summer.
I do not understand the "turbine effect." Say rotating individual buckets of 30w, 90w, 600w at the same rpm, I would expect the lower numbers to be thrown to the side to a greater, measurable degree than the higher. The idea behind using high viscosity 600w was that it would "cling" to the gears.