Oil on top layer of coolant Reservoir

Question

So recently when I got an oil change, they checked my other fluids and notified me that there is oil in my coolant... again.

Last year, I noticed oil in the coolant and took it to a mechanic, they checked everything out and said it was the radiator, so they replaced that and all the related hoses and the tranny cooler. Iv googled this, and it doesnt seem to be a common reason for oil in the coolant. But is it  possible?

This time I took it to a different mechanic and he checked out the head gasket, and the engine block and did all the pressure tests and everything checked out... but there's still oil in the coolant (a little thin layer at the top) so where is it coming from? 
My new mechanic thinks it could be left over oil residue from last time cause it's so hard to get completely out, but I'm not convinced? Can I siphon it out from the top, and see if it comes back? 
I drive  2007 cobalt LT

Pᴀᴜʟsᴛᴇʀ2 · Answer

Unfortunately you really need to flush the system in order to see if it's bleeding through back into your cooling system. There has to be a reason why it's there in the first place. It could be something as stupid as someone put some oil into the reservoir on accident, or might be an oil cooler leaking. There is oil getting into the system somehow. Flushing the system will tell you which way the problem lies If it comes back as much as it did before (or worse), something is seeping into the system. If it comes back but much less (or not at all), it was probably an accidental spillage. If you don't flush the cooling system, you're more than likely just going to see more of it down the road.

Answered by Pᴀᴜʟsᴛᴇʀ2 on April 21, 2021

Oil on top layer of coolant Reservoir

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