Mixing differential lubes?

[JSellers]

I'm preparing to change the axle fluids, and ordered a case of synthetic gear lube (I couldn't find enough locally at any one location, or of any one brand/weight.)

Now, with the lube arriving, and time for my garage/driveway work approaching, I'm wondering if a case will be enough, if I fully drain the differentials and axles completely.

With Mag-Hytec covers on front and rear, my documents tell me I should need 4 qts. front and 8 qts. rear. So, that will completely use the entire case of lube I ordered. What if that's not quite enough to fill both axles? Can I run down to NAPA and buy another quart of synthetic? Even if it's not the same brand of expensive 75W-140 that I just ordered and poured in my axles?

Am I shooting myself in the foot?

Thanks,
John