Ok- I really feel stupid for asking this question but here it is anyway. The gear oil that is in the front or rear differential- Is it shared with what gear oil that is in the axle? Or is the gear oil in the axle drained and changed independent of the differential. Seems like a stupid question but it is much cheaper for me to buy Valvoline 80w-90 conventional stock or Valvoline synthetic 75-90 myself and take it to my mechanic for the drain and change.