I would use a motorcycle oil, either regular or synthetic. In milder climates or when the weather is mild to warm or hot, Shell Rotella 15W-40 or Chevron Delo can be used. These are suitable for old motorcycles as they have the needed antiwear ingredients to protect the engine and transmission. Rotella has the Japanese JASO-MA rating, but probably any other heavy duty engine oil marketed for diesel engine use can be used, such as the Chevron Delo. The cold start number of 15W-40 may not be good for winter. I use either Shell Rotella T regular in the white bottle or Chevron Delo regular in my 81 KZ650-CSR.
note: modern car oils may not have the needed antiwear additives, especially ZDDP as it has been reduced or removed to protect catalytic converters.
1981 KZ650-CSR. Competition, Sports & Road.