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Yes, STOU viscosity is measured at engine operating temperature, hence the much lower index number than the gear oil, which is measured at room temperature. These oils are about the same thickness.
80w/90 gear oil is better though, because it absorbs less humidity than a STOU oil. STOU oils can take more heat. When used in a transaxle, the STOU gets warm enough to keep the humitity out.
80w/90 is best, 15w/40 will work too but will age quicker in a cold gearcase.