How often should you change gearbox oil? It's been over 40,000 kms and multiple track days since i last changed mine. I'm thinking i should do it again soon...

Open the drain plug and see what condition the oil looks, if it's still clean looking, I wouldn't bother. I would suggest around 50k before changing, no idea what the manual recommends though.

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Change it often. Maybe after every second track day. How little does it cost?  $100??
How difficult is it to remove and rebuild the gearbox?
Why leave old contaminated oil circulating in there?

I'd go 40k max on my understressed daily, much less on a 'performance vehicle'.

40k is generally toyotas recommendation for gearbox oil changes. As for a car that is tracked, once the oil has seen 120-130c it has started to break down. If your out doing 10 laps at a time, your quite likely to be seeing those temps. Normally I recommend clients do it every 3 track days

Your other option is to look into getting a used oil analysis done at a regular interval so you can monitor the condition of your oil and base your oil changes on condition.
Down your way you could get oil sampling done by Cavpower the Cat dealer for south Aus, If you contact Bill Tsoulakis (Btsoulakis@cavpower.com) he would be able to let you know the cost of their sampling but I would assume it would be around $30.
 
 

Thanks for all the feedback.
It all confirms my thoughts.
I've been very conscientious about doing engine oil and filter every 5000 kms, and replacing brake fluid every 12 months, but have been a bit lax with the gearbox oil. (and engine coolant).
I'll be rectifying this shortly :)