do some googling for more info on this. There are lots of write-ups on this.

Here's one:
http://volvospeed.com/Repair/AWD_bevelgear_diff_service.html

Drill the 6 o'clock stud position out as per the recommendations in the above-linked article. There is nothing for the drill to run into, so as long as you do it fairly straight and don't wreck the threads for the stud there's very little to mess up with this procedure. Use the correct size drill bit (outlined in the bay 13 guide) and cover it with grease to grab any shavings.

You can flush the gear out with regular/cheap 10W-30 motor oil too to make sure it's clean. Just reinstall the stud, fill with 10W-30, spin one of the front wheels back and forth a few times, and drain. Repeat a few times (should be able to get ~5 fills from a 4 L jug of 10W-30) and then fill with your real gear oil.

As has been discussed on this forum before, you will need a FULLY synthetic GL-5 gear oil. Should be easy to find. I recently read a rather interesting whitepaper on the AMSOIL Severe Gear product and will be using this from now on.
http://www.amsoil.com/lit/g2457.pdf
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1998 V70 AWD->FWD Turbo 200k+