Sounds like a Borton service rep. VW uses Haldex and the service interval varies between 20K and 50K, depending on make. This is the same Haldex that Volvo uses.
I would look around your area for a good shop and see what they say. Tell them you will supply filter and oil from Volvo.
Here is "a" procedure: http://www.volvoxc.com/forums/showthread.php?18722-Haldex-Filter-Repalcement

I don't know which generation Haxdex is in the 2004, probably 2nd generation, but even the haldex site has fliter and flush intervals as part of PM.

Come to think of it, I had Borton drill a hole in my bevel gear case ages ago so that I could flush the oil. Borton had never flushed that either! "we just replace the bevel gear case" with a rebuilt unit - for a small fee! Same with ATF.

Interesting reading: http://volvoxc70info.blogspot.com/2009/11/haldex-and-earlier-vicous-coupling-awd.html
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My back feels better when I sit in a Volvo seat