Does gear oil in a rear axel get thinner over time as it shears down, or does it get thicker with age as it oxidizes? I have seen both theories stated on this site. Also, FWIW my old (retired now) mechanic said never change gear oil, as it will cause the axel to start to whine. He was no expert on lubes, but had real world experience with servicing vehicles. He saw that when he did real axel oil changes, customer was soon back with a noise complaint. This would support the oil getting thicker over time, and then when new oil (thinner) installed the worn axel now has noise. What do you think???