AFPMSL.

Micky is referring to the oil level in the gearbox. It's like your engine, it has oil in it and it needs to be checked sometimes. 5th gear is separate in an RST gearbox, so unlike the other 4 gears it is possible that 5th can be fucked, but the rest of the gears still work, for a short while at least. Get it checked as soon as possible, but prepare yourself for a gearbox rebuild.