oil in coolant

I drive a 2000 Ford Focus SE with the 2.0 DOHC Zetec engine. There has been a light brown foamy substance in my coolant tank. From what I've read, it is probably oil in the coolant. There is no water or coolant
in the engine oil though. Anyway, yesterday I drained and flushed the system as best I could without a water hose. Anyway, I removed the coolant tank to get some of the gunk out and there was that foamy stuff in the three hoses connecting to it. What would be the best way to clean the foam out of the system? Should I use a bottle of that radiator flush? I believe the foam is restricting the flow of coolant. Would it be horrible to drain the coolant out, add the bottle of flush and fill with water, then drive it for a little while? I figured the driving would increase the pressure in the cooling system and possibly remove more foam. Any advice?
The cause of the oil contamination is still in question. I tried to pressure test the system at home but the tool didn't have an adapter to fit my tank. So I hope it is not a head gasket. I see a small leak around the thermostat housing that bolts to the head. Could that cause the contamination, or would it almost have to be the head gasket?
Well thanks for any help anybody can provide.
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Foamy? Is there a brown slime deposit in the coolant reservoir?
I had an S-10 with a radiator that was leaking and was corroded. Had the brown slime in the coolant tank. No water in oil, or for that matter, oil in the coolant. I think my 93 F-150 had the same symptoms, replaced the radiator... YMMD
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