Motor Oil is more expensive than Gear Oil, because of all the addatives etc. in there. So thats why they Recomend Gear Oil as normaly its more than good enough, there are Gear Oils i am sure off that are perfectly good for the bike but there a little hard to find and just as expensive as Motor Oil, so i just advice the 3 oils i got the best experiance with, compaired to others the clutch operates smoother and it shifts much better.
Fully Syntetic Fork Oil last longer so you can increase the service intervals, it might also be better for the fork it self.
If you replace a Piston there are a few things you need to keep in mind and are really smart to do:
A: Hone the barrel lightly.
B: Turn the Barrel upside down on a news paper/magazine on a table and than take the (new) piston with out the rings and drop it in there in the right direction, as soon as the piston drops past the exhaust port it should slow its fall a lot because of the trapt air, and it should gently come down onto the table with out a thud.
C: Fit a slightly bigger mainjet than needed for the first 250kmh to aid in lubrication and cooling of the piston.
D: sometimes replacing just the piston rings is more than enough, but do the test described in B: to see if the piston/barrel is still ok, but i find that buying a whole imitation piston with rings usualy doesnt cost (much) more than a set of rings.