As already stated here, Volvo has no service interval for the Haldex unit but the Haldex unit is used by VW in their quattro drives and VW recommend to replace the oil and filter every 40,000 miles.
Different generations have different set ups with the earlier Haldex units having a separate filter that needed to be bought and changed, the newer Haldex units have a moulded filter at the front of the pump that can be cleaned, if this filter gets clogged with dirt and the oil can't get through the filter then the pump will burn itself out.
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