regarding the subject of this thread, the factor that is of most relevance is that oil for gears is regulated by a standard created specifically for them, and differs dramatically from the standard for oil for engines: for engines says:. "in an example for SAE viscosity grades, lets say 5w25, the former grades denote a multigrade engine oil's low-temperature properties (W denoting "winter"), whereas the latter grades denote an engine oil's properties at the operating temperature of an engine''
the 1 for gears says: "SAE J306 is a standard that defines the viscometric properties of automotive gear oils. It is maintained by SAE International.[1] Key parameters for this standard are the kinematic viscosity of the gear oil, the maximum temperature at which the oil has a viscosity of 150,000 cP, and a measure of its shear stability through the KRL test."
and:Multigrade SAE gearbox oil grades include two numbers separated by a ‘W’ (75W140, for instance), with the initial number before the ‘W’ indicating performance at 0°C and the number after showing the lube’s performance at 100°C. As with monograde oils, the higher the number, the greater its viscosity.
hence the large difference of numbers that, if not knowing this, just seems incomprehensible.
and once again, thanks to the contribution of pizzaguy, who enlightened us with this valuable information.