The 8 hour oil change is important. The engine comes to you with 'break-in' oil in it. Do NOT wanna leave that in there longer than the 8 hour period. Or the oil filter that is full of that oil as well.
After that I change mine at end of each mow season, or @ 50 hours. Always with new oil filter too.
Remember you have (two) HG 2800 pumps on your Liberty that drive the rear wheels, & they require a change of both oil filters & the 20w-50 oil Scag recommends @ 50 hours. After that I believe next service for the pumps is 400 hours.
Enjoy your new machine. That is the same size Z I'm thinking about getting as well.