There may have been some oil in the coolant before the engine was worked on (assuming it was)
As a general rule, I would replace the expansion tank with a new one on the 603 engines when any top end work is done on the engine.
MB added a resin bed trap in it on the tank (according to gsxr) anyhow a new one will be nice and clean and if you then get some oil in it its a sign that would indicate a possible head gasket or (hopefully not!) cracked head.
And yes by all means, flush with DuPont or whatever citric acid stuff, then rinse three or more times to be sure its completely out of the system before adding 45% coolant/water mixture

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