Many factors can cause this rough or hard shifting.
Improper chain tension
To high an idle
Old oil in gearbox
Improper clutch adjustment
Worn clutch plates
Worn syncro's
An easy thing to do is to adjust yourshift lever to suit your foot position as all peaple are not the same size and we all do not position our feet on the pegs the same way. If you adjust the shift lever up or down you may find it easier to shift. Can't hurt to try it.
As for the question about oil......
Motorcycles can use standard motor oil. I do not ever recommend standard oil in motorcycle gearboxes though. The reason being a motorcycle gearbox is designed to be "constant mesh" the gears always meshed together. Now with standard oil your gearbox will breakdown the viscocity very quickly. If you use a good motorcycle gearbox oil with a higher sheer point you will not have this problem. Add to this the fact that a motocycle gearbox has the clutch bathed in the same oil making it do double duty. You can see that a good motorcycle lubricant such as Bel-Ray, Silkolene. Motul, PJ-1 etc are a much better choice than standard 10w-40 motor oil.