Isn't the factory service manual the authority on this? Here is what the FSM says for 4-speed transmissions:

"During cold weather, if shift effort becomes extremely high, transmission should be drained and refilled with Multi-Purpose Gear Lubricant SAE 80 or SAE 90, as defined by MIL-L-2105B or with automatic transmission fluid types labeled "Dexron Automatic Transmission Fluid or Chrysler Automatic Transmission Fluid AQ-ATF-2848A" available under Part Number 1843314. Automatic transmission Fluid should be replaced with Multi-Purpose Gear Lubricant SAE 140 in warm weather. NO OTHER LUBRICANTS SHOULD BE USED."

SAE Gear Oil 140 IS NOT EQUIVALENT in viscosity to SAE Engine Oil 140.

But SAE Gear Oil 90 IS EQUIVALENT in viscosity to SAE Engine Oil 50 according to this chart: