You are probably having a slightly different problem. Your head gasket has ruptured and is pumping oil into the cooling system. You should have this checked out and reapired as soon as possible. This mix creates a brown sludge that contaminates the entire cooling system and is extremely difficult to clean out.
The source of the problem is a cooling galley and an oil galley at cylinder #6. These two galleys are approx 1/2 inch appart. The gasket material in between the two galleys deteriorates over time and allows oil to seep into the cooling system creating the brown sludge you are seeing now.
92 Volvo 740 Wagon aka "Mutt mover"