I changed my Differential Fluid on my 04 Marauder at 18,000 miles even though the owners manual said to change it at 100,000 miles, I am so glad that I did it because when I took the differential cover off and let the fluid drain it was a greyish color, this was probably from the gears breaking in, my oil plug had a little magnet on it and it was covered with a metal glop, it might be beneficial for everyone that has a new car to change out the Differential Fluid earlier, at what mileage I do not know, maybe Pablo or Gary might chime in with a certain mileage recommendation.