Yesterday I changed my rear diff. Fluid and when I went to advanced to get the fluid I checked to see much I needed and was told 4 qt s. well I ended up buying 4 qt s of royal purple off amazon and while filling the diff back up I only got about 2 and a half quarts in and it full to the the fill cap. I'm sure this isn't the first time an advance auto parts dealer was wrong but I could swear I also read some where that I needed 4 qt's, but has anyone else changed there's recently and if so how much did you use? I guess tomorrow I could take the cap back off and check if its still full but I would love some others opinion. 2006 dodge ram hemi 5.7l quad cab 4x2.