I have an '03 TJ Rubicon and would like to change the gear oil in the front and rear differential. The dealership says that the oil is a new product and very expensive because of the airlockers. People have been running airlockers for years, so my question is what oil will work without voiding the warrenty? I was thinking of a full synthetic like Pennziol 75w-140 or quaker state. Thanks Jeepers!