GearBox Oil

My gearbox has a very slow oil leak. My Mechanic said not to bother fixing it as it would be very expensive. He said to just keep it topped up.
I was wondering if anybody knows how to top up your gearbox oil?
What type of Oil is best?
How much oil does a gearbox normally take?
Can I top it up myself or does it need to go on a hoist?
PS Its a Manual Transmission.

My 93.5 G20 took just shy of 4 qts. of redline MTL 90wt. gear oil. To do this you have to get under the vehicle then use a 1/2 inch drive to undo the drain plug for the transmission fluid to drain. The refill point is located on the front of the transmission facing the front of the car. I believe the refill plug is a 3/4 drive. not sure. I had to buy a $6 pump from autozone to pump the fluid back into the transmission this also had to be done from under the car. I hope that helps. Ohh yeah MTL smells horrible don't get it on your skin.

Cheers for the info. I shall give it a go