Yes they dont prescribe 10w/30 in heavy loaded gearcases, only in engine and rear end sump (transmission and hyraulics)
I dont know why actually. 10w/30 and 80w/90 are about the same viscosity, only 80w/90 is a gear oil, so the viscosity is measured at an oil temperature as is common in a gear case. 10w/30 is a multi oil, so the viscosity is measured at a normal crankcase temperature, about 100C
when 80w/90 gear oil, and 10w/30 are both at the same temperature, both will have the same viscosity.
As Hogi says, 10w/30 doesnt lubricate in hot conditions, 10w/30 can stand more heat than 80w/90 but lubricating meshing gears, is something totally different than lubricating piston rings, or slide bearings (hydrodynamic lubrication)
Am i right Hogi ???
oil seems easy but it gets quite complicated sometimes.