Just wondering, is it totally necessary to put in 75w80 as stated? Is there anything remotely wrong in using boggo 75w90 gearbox oil?

I wouldn't of thought so, and a few friends think the same, but my dad and a few of his friends, supposedly 'wiser' think its stated to be used for a reason!!

I use 75w-90 Motul synthetic gearbox oil in mine without any problems.

I think 75w90 is fine. Just don't use 80w90, makes for a horrible stiff gearchange. Found that out in my 205, trying to save a few £ by using cheaper gear oil

ECP and GSF don't seem to be selling gear oil via mail order anymore. Bit of a pain cos they did it very cheaply. Is 75w90 thicker than 75w80?

Yes.

I find the gearbox to like some oil´s and hate some.
Quaker State super Q5 works fine for me, but a "Texaco" gearoil (bought in panic at a gasstation at midnight before a race) with same spec. was worthless.

Redline is also known to be great, but for me it was a bit to much money.