Some OEM do recommend motor oil for manual tranny. All motor oil pass as GL-3 rating. Yan ang API rating for oil in metal surface protection. Yung gear oil are GL-4 or GL-5 rated more suited for moderate to high Extreme Pressure(EP) protection, moderate for SUV and high for racing and SUV towing. Creaful in choosing GL-5 unless specified, it can damage "soft" metals in tranny na gamit in older models-ie bronze
GL-3 is mild EP protection suited for cars for day-to-day driving. You can choose Motor oil for tranny and GL-4 90 as gear oil since gears carry more load in power transmission to wheels, around 30% more than that of tranny.
75w90 gear oil is about same viscosity as 15w-40 oil, about enough viscosity or "flow". Straight SAE90 is good enuff for gears

If you can use motor oil for tranny, can gear oil/MTF be used for engine, you may ask? No. Difference nila is in additive ie detergents. Motor have more of them since they are exposed to engine contaminants ie air particles 5 micron in size