Gear Stuck

[bokie]

cut story short. was at 3rd gear, drivin towards a red light, slowd down and try to bring gear down to nuatral, it didnt move, tried pulling it down , hiting it, it will not move, i step on my clutch a few times n still nothing move, push the car to side, and tried movin the gear agen. it did not move, and few sec after it just realease itself. and everything was fine. drove it back home, and at another light, same shiet hapend, i'm really p%$# off, went home, open up the panel and had a look at the gear box, i have a short shift kit btw, and the oil is abit greasy and gray/black-ish, is this normal? i've seen in other cars and its all clear, example this pic http://image-cache.skylinesaustralia.com/f...-1131976897.jpg

but mine is black, just wanna know whats wrong with my car,

ples help