The Platinums are definitely not what you want to use in this engine, but I don't think it has much to do with your problem. I had this same problem with my M103 engine. It turned out to be the cylinder head corroded between the water jacket and combustion chamber above number six cylinder. It happened due to lack of frequent anti-freeze changing by previous owner. I should have changed it when I first bought the car, bud didn't.
If this is indeed your problem, a competent automotive machine shop can weld up the head, and it would, of course, be foolish to not do a valve job, seals and guides at that time.
I'm not saying that this is the problem with your car, because I have not had the opportunity to diagnose your car. But, if the coolant has not been changed regularly, it very well could be your problem.
Testing the coolant for exhaust gas is definitely the place to start your diagnosis.
Best of luck,