Well about everything I have read people swear by Shell T diesel oil in 15-40What exactly brought you to that decision??
OK...I am going with Castrol Motorcycle oilCan diesel or automotive type oil be used ? Maybe...
To be used in a M/C the oil MUST carry a SG or SG/SH Rating... Not SH, SJ, SL, SM, SN...... It MUST BE SG ... Not CH, CI, Etc
The oil MAY have one of the other Ratings ONLY if it has the JASO MA or MA 2 Rating.
Best choice is to use a Motorcycle Specific oil.
The Best isn't cheap
Cheap isn't The Best