I always use the Honda oil GN4, or you can get the synthetic oil HP4S, along with the Honda oil filter.
My 2004 rubicon holds 5 quarts and you have to drain the oil in two locations (2 drain plugs)
What year is yours?
2004 Rubicon 500 4x4
Warn 4-2-4 kit
Front disk brake conversion
Deep lug mud tires
API Outdoors ATV rear rack organizer