Honda GL1800 2004 Owner's Manual page 134

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 motorcycle 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.
Engine Oil & Filter
SG or higher except oils labeled as energy
conserving on the circular API service label
SAE 10W-40
MA
Pro Honda GN4 or HP4 (without
molybdenum additives) 4-stroke oil (USA
& Canada), or Honda 4-stroke oil (Canada
only), or an equivalent motorcycle oil.
OK
Servicing Your Honda
127