1981 Honda CB750F supersport

[milesdriven]

Just purchased a 1981 Honda CB750F supersport, and I'm wondering what would be my best choice for oil. The previous owner had it serviced at a shop, and is not sure what grade was used. I don't have a factory manual, but the bike recommends 10w40. Any suggestions?

 

[XS650]

A 15W-40 HDEO that has the Allison C4 spec on it.