How much differential and gear oil do I need to do the front, rear, and gearbox in my 00 OBW 5 speed?

All listed in the back of your owners manual. In other words, RTFM!

The rear diff takes 0.8 liters, if totally drained. The front "transaxle", as I like to call it since the diff and gearbox are the same unit (assumming manual shift) will swallow 3.5 liters.

 

In my experience, the transaxle takes 3.3 liters to fill up, and the diff around 0.7 since you cannot drain all the old oil out.

 

That means you need to purchase 4 liters.