That could be an old timers trick, when the widely available gear oils were pretty thick.
Believe it or not, the viscosity of a grade 30 engine oil fall within the viscosity ranges specified for 80W and 85W gear oils. There are gear oils available now in a 75W-80 grade, which are thinner than SAE 30 engine oil. And they have all the extreme pressure additives engine oils don't. So if the idea is to use a small amount of thin oil in pursuit of that last .001 of ET, these might help the rear end live longer.
