I have a 2003 C4S with 35k miles and am changing the transmission and front diff oil. The transmission oil looked good and didn't any issues. When i removed the panel under the front diff i noticed some greasiness around the diff but haven't had any drip in my carport. I drained the oil and it definitely wasn't as clear as the transmission oil. The drain plug also has quite a bit to metal around the magnet. At a minimum i plan to change the seals so it doesn't leak. Do i also need to have the diff rebuilt? It isn't making any noise yet.

 

I’d drain that oil down then put some fresh stuff in and see how it behaves. It’s easy to change out if it comes to that, but probably not necessary at the moment. In fact I’m sure a few folks will chime in and say just take the front diff out and leave it that way... It’s SOOOOOOO much better that way...

 

My front diff began leaking and making a very noticeable whining sound when off the throttle before I bought a low mileage used one from a RL 996 turbo owner who had removed it to go RWD. You will know when its time for replacement. They can be rebuilt for about $1100 and used ones should be 1/3 that cost. New ones are just stupid expensive and not a realistic option. Best of luck.

 

That metal residue on the drain plug doesn't look bad for 35k miles, especially if it's the original fluid in there. Change it and you'll be fine. Changing the axle seals is a good idea though since they're pretty inexpensive and easy to do.

 

Thank you for the feedback. I still am not sure if the differential is leaking. Although it was pretty greasy under there i drained right at 1.5 liters out of there so am just refilling it for now and will keep an eye under there. I did notice one of the boots on the CV joint was split so have ordered inner and outer boot kits and will replace this weekend.