You can't compare motor oil ratings to gear oil ratings. 10w-40 motor is equivalent to 75w-90 gear oil in viscosity.
The primary difference in gear oil and motor oil are the additives. Gear oil has to resist the shearing effect of hypoid style gears slicing across each other. Motor oil has to resist the crushing forces on the bearings when the piston changes directions in the bore.
Another thing to keep in mind with gear oil in a manual transmission. Most of the gear oil you will find at the parts store has a GL5 rating which is great in a rear end but the additives used for the GL5 rating is corrosive to brass syncros used in manual transmissions. If you want to use a gear oil look for one with a GL4 rating or better stay with a manual transmission fluid.