Gearbox death throes?

[gnt@nhm.ac.uk (Graham Thackrah)]

I may be in the market for a new car soon or at least a large outlay of the
green folding stuff as I think my gearbox could be about to develop
something nasty. The shift feel has changed quite a lot and is a lot more
notchy than it was. I have bought some gearbox oil as last time it felt like
this was just before a new clutch and top up for the gearbox oil, it
required 500ml in a box holding 2 litres. The problem was cured that time so
I am hoping that it is as simple as that. Of course it would mean that my
gearbox has a leak but I can cope with topping it up regularly. Why would
the shift feel change if the box was low on oil? 

Also there is a whopping allen fitment to remove the filler plug, where on
earth do I find very large allen keys or allen socket fitments, I am not
sure of the exact dimensions but it is way over my biggest key of 3/8 inch.
Is it also cruical to tighten the filler plugs to the designated torque? It
will be tricky getting a torque wrench into the space. 

I was also thinking of replacing the oil in the gearbox, the garage said
that is was not necessary but I can't see it would do any harm and may even
remove any bits of metal that have surely accumulated over 111000 miles of
crunching gear changes so they dont act as an abrasive to what is left of my
gears.

A friends identical car is off the road at the moment with a very deceased
gearbox, it has completed almost exactly the same mileage, is this too
coincidental?

Thanks in advance,

Graham.

'85 80 sport.