Like lincster said, it takes 13 qts if you want it filled up. Anything less and I would feel you'd have to check it a lot more often eventually to make sure you're not low.
I've always used what the manufacturer recommends and I've never had issues...think I'll stick with that.
2011 F350 CCSB, 4x4, 6.7L, 3.31 E-Locker, MBRP 5" DPF Back, Innovative Torq 2
2012 Ram 3500 CCSB, 4x4 6.7 Cummins, G56 6 spd
2011 F-250 CCSB, 4x4, 6.7L, 3.31 E-Locker
2009 F-250 CCSB, 4x4, 6.4L,Torqshift, Spartan
2009 Dodge 2500 QCSB, 4x4, 6.7L Cummins, 6 spd,
2007 Chevy 2500HD Classic, CCSB, 4x4, Duramax LBZ, Allison 1000, 4" MBRP Cat Back, Diablosport Predator, Edge Insight
2005 F-250 CCSB, 4x4, Torqshift, 4" Cat Back
2004 F-250 CCSB Fx4, 6spd, Bully Dog Intake, Banks 4" turbo back, Edge Juice
2002 F-250 SCSB, 6spd, AFE Intake, ATS Turbo Back Duals, B&D 60hp chip