The closest you can get to a one fluid does it all is CAT TDTO or TDTO TMS which are TO4/TO4M spec fluids which are gear/hydraulic/final drive fluids.
IIRC TDTO comes in straight grades (10W, 30 and 50 ?) and TDTO TMS is equivalent to a 15W-40/75W-90 oil.
The TO4 spec is a really, really stringent spec but I'm not sure it'd even be beefy enough to protect the 9" properly, you still need a dedicated diff oil for that.
I think even CAT use dedicated, heavier final drive fluids for those machines that have separate compartments.
I'm going to buy the TDTO TMS for my old Fiat tractor which specs engine oil for everything.
An oil bloke from Sth America emailed me a chart from a CAT study showing the massively reduced pinion wear on CWP's from using the SAE 30 spec CAT TDTO vs a good 15W-40/GL-4 dual rated engine oil. (remembering tractors use centreline spiral bevel CWP's, not massively hypoid like a 9")

