oil in water fill tank on 2002 Saturn LW300

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i'm getting oil sludge in my fill tank where i put the water in. but if i drain the radiator the water is clear. i just changed the oil and it was perfect.I'm stumped.
(2) Answers
Your problem is the oil cooler itself, it sits in the top of the block between the pistons. The cooler gets pinholes in it and leaks oil right into the coolant passages. The reason your coolant is clear is because oil floats and colects in the resevior. The part is a little over 300 dollars from saturn but will fix your problem. I believe its about a 4 hour job. The intake has to come off. You might also have a blown head gasket but thats not what is causing this.