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I do notice that my gearchanges are quieter when I start out on cold mornings, presumably because the oil in the gearboxes is thicker.

I phoned up and spoke to the Chap at Nova Racing. He advised just normal red transmission fluid, so that's been my choice for now.
Did run in a breather tube which runs uphill from the connection stub. The theory being any oil that gets pushed, expanded, blown or whatever will gravitate back down into the box.