loose nut

Normally I use the ac delco 10-4061 88863089 gear oil with delco 10-4003 additive in my c3 differentials and I have never had an issue with clutches popping or making noise. I ordered 2 qts last week and I was told that 10-4061 has been discontinued and 10-4108 is the gm recommended replacement oil. The 10-4061 was mineral based and they have replaced it with synthetic 10-4108. I have just rebuilt my differential in my 76 and now I am trying to figure out why Eaton says not to use synthetic oil. I have read so many posts with people saying that you should not use it but no one ever explains why or what issues it will cause? Years ago I had regular 90wt + gm additive in my 79 and it had clutch popping issues. When I changed to ac delco oil w/additive it went away completely. I have heard that synthetic is too slick - but then aren't we adding the additive to increase slippage and keep the clutches quiet? Zip and Corvette Central are both showing the synthetic 10-4108 when you search for c3 differential oil.
In short Eaton says don't use it and gm offers it as a direct replacement for the correct oil that they discontinued. Can anyone tell me what issues that you have had after years of synthetic oil use? Is anyone else using AC Delco 10-4108 and are you having any problems? Thanks.

69L88

I would simply recommend you get some Lucas 85W-140 and 2 bottles of the 10-4003 and sleep well. No need to boil the ocean.

GTR1999

I don't use GM oil, I have used the Lucas 85-140 for the past 20 years, change it at the first 500-mile break in, again at 8-10k miles. I use the GM additive but 2 bottles.

I never use the spring packs or fiber clutches unless requested. I don't like either and I know how to tune a posi without them.

If you run fiber clutches, you can probably run any type of oil. With the old style solid steels, the synthetic oil may cause chattering so plain old dino gear oil works, with the additive, and correctly setup posi.