Gearbox, oil top up?

[serious tom]

My SR20 gearbox feels more like using a toothpick than having a fluid motion like say a vw...
Third gear is an issue, have to slam it in.. Possibly the syncro?..

The rubber boot looking thing that bolts onto the tunnel is ripped, and ive smelt a gas type smell before, which is gearbox oil.. A little bit has leaked out...

Can I just top it up?
Would this help things?
And how would I do it,?

Thanks,
Tom.

[2tonne]

The filler plug on the side of the box is also the level the oil should be. .(fill till you spill)

[serious tom]

any specific oil I should be getting?