2004-2005 660 Yamaha Rhino Factory Service Manual, Page 111Get this manual
CHECKING THE DIFFERENTIAL GEAR OIL CHANGING THE DIFFERENTIAL GEAR OIL
3Check: oil level Oil level should be up to the brim of holeOil level low Add oil to proper levelRecommended oil SAE 80 API "GL-4" Hypoid gear oil CAUTION: Take care not allow foreign material to enter the differential gear case4Install: oil filler plug