What kind of oil should I use?

Your dealer is correct(technically speaking).Let me tell you why.Automotive oil,works fine in the enviroment in which it is designed to,which lubricates the piston,crank,cams,etc.....none of these put extreme pressure on the oil molecules.However,on motorcycles that have a transmission that shares the same oil as the parts listed above,the oil is placed under extreme pressures when it gets squeezed in-between the teeth of the gears,so much to the point that normal motor oil molecules will shear,as in tear.This is why motorcycle oil is engineered differently from automotive oil.So,you can use the automotive oil,just know that the automotive oil will breakdown molecularly much faster than motorcycle oil.