My owner's manual doesn't specify a change interval for the differential fluid, but at 78k I figure it's time. I have an 09 4wd Liberty with a tow package. The manual calls for 75w140 synthetic in the rear and 80w90 (dino or synthetic) in the front. I am a by the book kinda guy, but on the jeep forums the consensus was that dino oil was better in the differentials than synthetic. Also, the feeling was if the vehicle wasn't being used for off road or towing (which we don't, it's my wife's jeep) then 80w90 was fine for both front and rear. Curious as to what the BITOGers think.