For the diff you'll need to get 2 litres of 75W-90 gear oil (capacity is around 1.5 litres). I used this stuff in mine:
Fuchs TITAN GEAR SYN 75W-90 Semi Synthetic Driveline Oil
There's a tutorial here: http://www.lexls.com/tutorials/suspension/diffchange.html
The gearbox uses a completely different oil which you can only get from Lexus/Toyota. Ask for Type T-IV automatic transmission fluid. I paid around £50, can't remember if that was for 4 or 5 litres. You need about 2 litres to drain the sump and refill (this isn't a complete fluid change, that's a bit more involved). There is a filter inside but I don't think it's designed to be changed often.
Again there's a tutorial on the LexLS site (for Mk1 & 2 but Mk3 should be very similar): http://www.lexls.com/tutorials/transmission/transchange.html