Reduction gear oil

My Terex 760 calls for 85W140 in the front and Esso Unifarm in the rear. Two questions: 1. The reduction gear oil is not spec'd. I'm assuming it is the same as the axle oil, correct? 2. Esso Unifarm only is spec'd for the rear, and I have found it to be made in only 10w30 and 15w40. 15w40 is a good all around choice, correct (I won't be using this much, if at all in subzero temperatures)? Thanks, John