share more details. u mean that shifter shaft can go to 5th gear position but it does not lock into 5th gear? or that it is restricted to go in that position.
anyway to me it seems like a loose cable or worn bushing. you should get cables and linkages checked by 1,2 mechanics,
to check gear oil level, remove gear oil fill plug(screw) it is directly under distributor, at front side of car on gear box. you need 24mm spanner to open it. bigger screw that is popped out is reverse restrict pin. don't open it. after opening, put a wire inside it while holding other end. DON'T let wire fall in gear box. from wire u can know that how much oil is left. if oil is black, drain it and refill new oil. other wise fill using a tube and funnel till oil starts to drip out) i prefer zic FF oil 75w-85.
so do these two things and update us.