As an aside, I have noted many, many products that purport to increase fuel economy. Magic oil additives, intake swirl-inducing inserts, multi-tipped spark plugs, and uber-super-dooper CAIs all increase fuel economy. Amazing! If you add up the percentage increase in fuel economy of all of them, it leads me to believe that if you bought and installed them all you'd have to offload excess fuel or risk rupturing your fuel tank!
BUT this is unlikely in the case of a front differential. My guess is that there is a layer of water under the lube. Verification of that should lead to an inquiry into how that much water got in there.