I took the Z out for the last spin of the season before putting it up for winter hibernation.
Temps were in the upper 40's.
The cruise was about four hours long which consisted of about 2 hours highway speeds and 2 hours back roads.
Ten miles from home, I filled up the tank and added some Stabil.
I got home & lifted the car for an oil change & removed the wheels to clean the brake dust out from inside the rims.
I then climbed under the car for my normal chassis visual inspection. At the rear of the car, I discovered a dime sized drop of oily substance on the garage floor. It smelled like differential fluid.
I then observed the driver side rear aluminum X-member (aft of the rear spring) dirty with a damp oily deposit.
The rear edge of the spring was a bit oily as well but not too extensive.
Upon further inspection, looking up from the floor, I noticed a small green oil drip on the lowest gold/brass colored small bolt head located up between the rear of the spring and aluminum X-member on the driver side diferential cover. I observed that most of the oily residue was caked up on the black stamped bracket and rubber isolator that I assume to be the differential mounting bracket located aft of the differential housing. The aluminum X-member aft of the rear spring had oil and dirt caked on it. There was a very small amount of oily deposit on the highest upper curved portion on the drivers side exhaust tail pipe.
I did not see any oily deposits around the axle shafts where the go into the differential housing. My focus was trying to see it it was leaking from the ever so common C5 driver side rear axle shaft/differential seal problem. It did not seem like that was where the leak was coming from.(but I could not see up high between the seal and body of the car) The differential housing seemed clean and dry, as did the transmission. Again most of the oily build-up was on the components aft of the rear spring.
I know driving at highway speeds causes alot of wind turbulence underneath the car and can make diagnosing where the leak is coming from difficult.
I noticed that after doing a search here on the forum and reading various posts, many of the cars with the rear differential axle shaft seal leaks had much more rear lube deposits splattered everywhere including the differential housing. Perhaps mine is in the begining stages.
I just want to get some thoughts about this before I start shopping the repair around next spring.
Any input would be appreciated.
Thanks in advance,
Silverbird

 

If it's not a leak from one of the diff seals, here's one possibility. If you haven't done so already, reach up on the left top of the diff and see if you can find some diff oil around the vent cap. It may have been overfilled at some point, built up some pressure and released through the vent whenever it reached a certain point.

Here's a pic of a diff with the vent showing as a small, silver cylinder on top.

 

Thanks for the input!
Went out and checked.
Felt arount the vent.
Seemed quite dry around it.
Looked closer at the driver side differential snout near the axle seal.
The cast housing seems a little damp in the area including some of the cast ribs.
Not much, just a little bit.
Looking more and more like the dreaded driver side seal with the diferential lube getting blown back at highway speeds.
The begining stages anyway.
Thanks again!
Silverbird

 

That's a pretty easy diy fix. You can get the seals from Gene Culley at his GM Parts House web site and diy on jackstands in your garage. I can email you the procedures if you decide to tackle it yourself.

 

Each time I experienced the leak, I noticed oil spots on the rear of the car. That was always my first clue that the leak had started again.