hello, what type of oil is used in the gearbox of a fabia 1.4 16valve 100bhp

probably something like 75w90 oil , although i'm not too sure on that

Try G500 it fully synthetic.

I've just done my car with Millers TRX Synth 75W-90.

It is a fully synthetic GL-4/GL-5 oil and also costs significantly less than the dealer stuff too

Has changing the gearbox oil made any noticable difference?

On the Octavia, yes it has, however it still isnt' perfect it is a lot better and has only done maybe 150 miles since changed.

Have a look at the gearbox oil thread under the octy section.