Can we go back a few steps here? So far, the two things you've told us in terms of what you consider to be a bad head gasket are that you think the cylinder pressure is pressurizing the cooling system, and that there is head gasket seepage.

For the first, did you ever hook up a coolant pressure tester? These can do two things; you can leave it connected and warm up the engine. This will show if you have excessive pressure. Second, you can pressurize the system while it is cold, and see if it holds pressure. This is a MUST DO device to properly diagnose any problem with the cooling system. The Uview can also eliminate the cylinder gases getting in the cooling system as a possibility, but it's nice to know if the system pressure is right, and it can also help find leaks, and determine if the system holds pressure.

For the cylinder head seepage...are you actually seeing it leak from the joint of the block and head? Or are you seeing white smoke in the exhaust? Or are you just losing coolant? Are you getting coolant puddles on the floor?

Using stop-leak on a system like this is bound to cause all sorts of other problems. The sooner you flush all that crap out of the system and address the problem the better. These cars can hardly even tolerate the wrong antifreeze; adding stop-leak is really not good.

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2014 BMW 328i Xdrive
2011 BMW 335i M-Sport
2008 BMW X5 4.8i Sport
2000 BMW 528i 5sp