Honda TRX420TM Owner's Manual page 116

Engine Oil & Filter
Oil Recommendation
API classification
viscosity (weight)
suggested oil
Motor oils intended for Service SE, SF or SG will show this
designation on the container.
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
106
SE, SF or SG
SAE 10W-40
Honda 4-stroke oil or an equivalent
OK