Very little - the S has a few cosmetic fairings and the N has a different dashboard that moves with the handle bars, but that is it.
Read the user manual of bike and then concern your mechanic.
34ftlb is the torque for 1993 Yamaha xj600s. I am giving the linkthat will increase the knowledge about bikes.
20W40 with no friction modifiers is what is called out for my XJ600s