Auto Parts: Maintenance Question, limited slip differentials, wheel drive car

Question
How often should I change the rear axle lube in my rear-wheel-drive car?

Answer
    Well, in the old days we never changed it.  Nowadays I would say to look in the owner's manual for advice on that subject.  There are a couple of exceptions, though.  The differential housing has a vent in the top.  Jeeps have a hose that extends upward in the chassis so that the vent won't get water in it unless the vehicle is almost totally submerged.  Other vehicles don't have this.  So, if you are ever force to drive through deep water, you should change the gear oil immediately afterward.  Gear oil reacts in funny ways to mixture with water, none of them good.  Another exception is in the case of limited-slip differentials.  They should have the lube replaced at much more frequent intervals.  I'd recommend 15K miles.  Limited-slip axles require a different lube than regular differentials.  And, in many cases, they also require a special additive in addition to the lube.  If you are working with a newer car, I would recommend buying both the lube and additive from the dealer.  Make a service note in your service records, and keep the receipt from the dealer so that you can prove you've been properly servicing the differential.  Differentials, especially limited-slip differentials, are very expensive to replace.