Honda TRX420FA 2009 Owner's Manual page 120

Engine Oil & Filter
Oil Recommendation
API classification
viscosity (weight)
JASO T 903
standard
suggested oil
＊
Suggested oils are equal in performance to SJ oils that are not labeled
as energy conserving on the circular API service label.
Your ATV does not need oil additives. Use the recommended oil.
Do not use oils with graphite or molybdenum additives. They may
adversely affect clutch operation.
Do not use API SH or higher oils displaying a circular API ''energy
conserving'' service label on the container. They may affect
lubrication and clutch performance.
NOT RECOMMENDED
Do not use non-detergent, vegetable, or castor based racing oils.
Servicing Your Honda
112
SG or higher except oils labeled as
energy conserving on the circular API
service label
SAE 10W-30
MA
＊
Pro Honda GN4 4-stroke oil (USA &
Canada), or Honda 4-stroke oil
(Canada only), or an equivalent
motorcycle oil.
OK