I told my mechanic to change my differential oil. He inspected it and "saw there was no water in it" and told me it's a waste of money to replace it. He told me if it was his vehicle, he wouldn't do it. I asked him how often he recommends I replace it then and he said every 30k to 40k miles. This mechanic works on all vehicles and does not specialize in 100-series vehicles. He's a good mechanic and an honest man. I'd like to hear from somebody mechanically inclined on here on whether they agree with my mechanic or recommend I replace it every 15k like the owner's manual says. Additional info that may help: I drive the vehicle an average of 4,200 miles per year. It's all on-road driving. No off-roading or towing a trailer (I plan to start towing a light-weight trailer approximately 2 miles per week).