Oil in the coolant means either a cracked head or a leaking head gasket.
I've not heard of the 603 head gasket leaking coolant out the side, but when composition gaskets (rather than the old asbestos/copper ones from days of yore) fail, they usually blow at the coolant or oil passages -- the sealant applied during manufacture goes bad, and the coolant in particular will dissovle the gasket material.
That means it can easily erode it's way out to the side of the block and dribble.
Find a mechanic's mirror at the auto parts store -- they are usually about 2x3" on a swivel on an extension rod -- and see if you can use it to peek under the exhaust manifold. Should be obvious if you have coolant leaking there with engine hot.
If you still have oil collecting in the coolant tank, chances are a cracked head, but that won't leak externally, you will just have coolant dissapearing. The good news is that if someone fixed either a blown gasket or a cracked head in the past, the oil will hang around forever, even if you flush quite a bit, so traces are not a panic item unless it keeps accumulating!
That's the story on my 300D -- oil in the coolant tank when I got it, was planning on a new head, but no more has appeared, and the rad is brand new, the head is suspiciously clean......probably the secont (or third!) owner got bitten for a new head.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!