Hi, it you`ve run the adjuster nut up as far as it will go then a new cable is going to be needed anyway
sooner rather than later. It could still mean a gear box out job for a load of reasons, but a new cable will
be a starting point. For the oil you may have to remove a gear box cover thats held on with pop clips..or
cable ties if someones been there before and had trouble

If I remember taking the top half off lets you
at the oil inspection plug, just fill till it shows here then you know that it has plenty oil. If you want to drain it
and go new oil it will take over a ltr but less than two (unless you forget to put the drain plug back in)
Assuming it`s an mt75 box , all the knowledge on here is to use the specific oil, handily called mt75...e-bay
Halfords or your local factor. If this simple solution doesn't work them you will get plenty help and advice
as you go up the difficulty gears..

so to speak

Good luck.

An engineer can do in an hour for a fiver what any damn fool can do in a day for £100