I did a search outside of this forum and came across a thread from this forum from two years ago which was amusing but one I recall now from a trusted member who had himself had to add a quart on his brand new truck to bring it up to near the level plug level. Then after accumulating 5000 or so miles on the truck he pulled the cover to drain it out and wiped out the bottom of the diff housing and put the factory cover back on and in his case it took exactly 4 quarts to bring it up to within that 4/10 of an inch below the fill/level check plug, far more than the GM spec. That is a big difference between that GM spec ( which is why the GM diffs are below the level they themselves say they should be when coming out of the factory ) vs what amounts to around 8 pints of oil.
My own truck when I first got it home from new, I had not measured how much volume mine took since I used a hand pump on a 6 gallon pail of gear oil to top it off but I had measured it by using a bent wire on the flat shop floor after the truck sat overnight and a ruler and it was exactly 1" below the bottom of the fill hole on the rear diff. That fell right into line with what others have measured from a factory filled rear diff on these HD trucks, however the GM instructions are to fill from within between 4/10" of an inch or up to the bottom of the fill/level check hole as the full range. So that is why it alarmed me when you mentioned not seeing any oil in that window although I wasn't sure if that diff cover held the exact same amount as the factory cover so can only speculate its probably very close to retain the same oil flow characteristics over the top that the ring gear provides with the factory curved cover vs some of these odd flat back diff covers ( the whole video Gale does on that about some of the goofy after market covers out there ).
I agree it's ambiguous, and somewhat complicated with a shared application. For S&Gs, I just looked on Amsoil's website for a 2023 Ram 2500 with 6.4 gas. https://www.amsoil.com/lookup/auto-and-light-truck/2023/ram/2500-pickup/6-4l-8-cyl-engine-code-j-esb-j/
There's a lot of listings for Ram rear differentials, and too early in the morning to dig into the variants. But there's a big difference in fluid volume called for between them and GM applications, assuming it's the same axle assembly.
So, if GM differentials tend to be underfilled, what does everyone think about a 7 pint fill, as opposed to the 6.3 pints listed on Amsoil's website? It would somewhat conservatively split the difference between Ram and GM specs.
Thanks for the reply’s everyone. I will try to find the alignment printout and post that up. Sounds like I’ll be taking it back to have the alignment re-checked
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