When I installed my lockright in the rear of my 86, the manual for the locker said to use 85w140, which is what i used. I dont know if it is from the real thick gear oil, or a combination of the oil, new 31 muds, and locker; but my gas mileage has gone from 330 a tank, to 230-250. I realize there is a slight difference in the speedo now, but only a few %. Will changing the gear oil to 80w90 effect the mileage at the cost of increased noise, or should I just leave it. I live in AZ, so the temps are higher than other places if that changes anything.