check the oil again and see if you can smell gas, that should also indicate a head gasket might be blown...

seems funny that this started after you put the other additive in it.. maybe you are actually seeing that stuff.... why not try
draining the coolant and oil. replace and without the stuff you added and see what you get... might be worth a try...

also if you have a blown head gasket, that booger should be running hot pretty quick, water out of the tail pipe, loss of coolant....I am far from being a mechanic but that would be what I would do or have a good mechanic you trust to check it out..

good luck, let us know what you find

Fred

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