Manual has to say something. I'll give it another look before I go out today, but 8lbs sounds good to me.
Manual says:
2010 RZR and RZR S 8 psi front and rear
2010 RZR 4 10 psi front 12 psi rear.
I know the question was about the S, but just an Fyi for people suggesting tire pressures...
This came up awhile back in the RZR 4 section and people with 2 seaters were chiming in with "6psi", "8 psi" "never more than 10" ect....trust me with the extra weight(300#,not to mention the possibility of 2 more passengers) of the 4 it needs more tire pressure than the 2 seaters in any given scenario...
Exact tire pressure to an extent is personal preference..trails I run more,less rolling resistance and more wheel protection...with obstacles,mainly downed trees for me you have to balance wheel protection with the need for traction in that case i run a little less than normal trail pressure but not as low as I would run in the sand. mud and especially in the sand I air down quite a bit for traction,wider foot print. With the stock wheels you just have to remember you are running less air and be a little more careful not to pull a bead