I need to change my differential fluid and cannot find the correct replacement.  My 1973 has the original fluid from it's build date.  It has 56,000 miles.  My research found the following: The correct fluid is ACDelco 10-4061 (88863089) which has not been manufactured for the past several years.  It's a GL-4 conventional gear oil. The replacement ACDelco product, 10-4108 (19352759), is a synthetic GL-5 product that has higher levels of additives that are corrosive to the soft metal parts used in older transmissions and differentials.  I should note that the 1973 Chevrolet service manual calls for GL-5 fluid, but that was when GL-5 fluid did not have the higher amounts of additives now used. I just changed the transmission fluid and after much research decided only DRIVEN conventional racing gear oil 80W-95 GL-4 or RED LINE MT-85 GL-4 (synthetic but formulated for soft metals) should be used. I went with the DRIVEN and wonder if it could be used in the differential along with ACDelco limited slip additive, 10-4003 (88900330). Any knowledgeable recommendations would be appreciated.  BTW, if you look on Amazon for 10-4061, they show a picture of it but ship you the 10-4108.  I already went down that rabbit hole. Thanks, in advance, for responding and sorry for the long post.  All the previous posts on C3VR are almost 20 years old and not relevant today.

Hi, I have a post on my thread concerning it. Should be 4-5 years ago. But, I used two bottles of AC Delco limited slip ADDITIVE. Then topped off with Penzoil 80-90 gold. It is recomended for soft metals, and works well on brass synchronizers in M22 and 21. Drove around in figure 8s for 15-20 minutes to lubricate the clutches. Been quiet and smooth since added. 

ANY GL4/GL5 gear oil is fine for the diff, or manual trans in our cars. The diff requires the posi additive, obviously. Sometimes online info can get a little too convoluted, and misleading....but then again, the info here is also online, so....😗

Been doin transmissions/drivelines for goin on 50 years, and gear oil is gear oil when it comes to these old turds. 

Phew! 😄 I was glad to read the post from Adam's Apple - I use a "Classic" diff oil in our '78, from a UK company that is well renowned for it's classic (ie. mineral) oils and fluids.  But I checked after reading the initial post and see that it is in fact GL5.  But like Joel, I use two bottles of additive each time I change the fluid in the diff, which is usually about every 6 years or so.

What I would say is that if the oil has been in the car since new, you're going to need a mask or breathing apparatus when you drain it because it is going to STINK!😷🤮😀

Thanks to all who responded. I'm extremely picky when it comes to my car and I ended up calling and speaking with C&S Corvettes in Florida, Yogi Bair's Corvettes in Pennsylvania, Corvette Care in Ohio and Driven Racing Oil in Tennessee and learned the following. First, got confirmation that these old gearboxes and differentials do contain soft metals, like copper and brass, and not to use any of the modern GL-5 lubricants that have all the additives.  It's best to use a GL-4 conventional lube or if you want a synthetic, a GL-5 that is specially formulated for soft metals (like Red Line MT-90). Bair's told me they use Pennzoil 80W-90 Gear and Axle oil, which is a GL-5 conventional oil but I could not find anything on Pennzoil's website indicating it was ok to use in applications where soft metals were used. Driven was the most knowledgeable about this issue and indicated that the same oil I used in my transmission should be used in my differential with the addition on one 4oz bottle of ACDelco Limited Slip Axle lube (10-4003). As noted in my original post, I used Driven conventional racing gear oil 80W-90 GL-4 in my transmission. It notes right on the bottle that it is safe for all yellow metals. 

Again, thanks for all who responded and I hope this information is helpful to others.