For some twisted reason, capacities involving "hypoid oil" like differentials and manual transmissions, used to have capacity specified in PINTS/PT. 12 PT is 06 QT.
That said, are you sure you have a Dana 135 Axle? That bad boy has a capacity around 12 Quarts, but seems to be used in bigger trucks.
Dana axles are ID'd by Bill of Material (BOM) Number, a tag on one of the cover bolts. Did you find a tag, what did it read? The info I looked up, on Dana, for 2006 F-53 came up with an axle designated "80/286"
I used
This DANA Page and entered Ford 2006 then pulled down to F53. If you can find BOM, enter that.
I think you're right to use 75W-140 full syn lube. I went through this project on our 2002 E450. The manual doesn't say that, but current practice is to use what you chose, same as I did. Going to that lubricant, makes it permanent according to Ford, unless you're running (go figure) an Ambulance.
This ID Page may also help you confirm what you've got so you can proceed to verify that you put enough lube in it.
I was going to drain our axle by removing the cover since it didn't have a drain plug. A tech I trust, said "Suck it out with a Slurp Gun." That was when I remembered the "Oil Boy" I use to suck the oil out of our V8 boat engine through the dipstick tube. I waited till we came in from a 100-mile run, and pulled the axle lube out with Oil Boy. Measured what came out. Matched the spec. Filled with 75W-140 Supertech from Walmart. Took the amount shown in the spec and what I'd pulled out. If all you have is "clingage" from the original lube, no worries. And it sure doesn't sound like you somehow got only half the old lube out before you filled it with new.
