I get this on my 1996 850 GLT with 305k on the motor for the last few years during the cold weather here in CT and then it goes away when the weather warms up. Klaus may be correct in that the oil seprator lines are backed up but I have had no damage to speak of for the last 3 years(unless this is part of the reason why my crank and cam seals have finally blown on mine, but I think that problem is more related to a backed up pcv system which we are replacing along with the seals). The Volvo dealer did not seem to think that the crud had any relation to the blown seals. So I would continue to do your normal service and oil changes and keep the flame trap nice and clean and not worry about engine damage or trying to drop the oil pan and clean things out. Not worth that effort IMHO. You might ask your dealer or mechanic if he thinks it is related to a gummed up oil line or the pcv system.

Marty