Common to every m series BMW engine, especially m52s that see extended times at high revs. overfill the oil by half a liter and the noise will go away or be significantly reduced. Don't change viscosity and don't worry about it. Mine made the same noises and after two summers of weekly track use the cams and lifters still looked minty. stick with a very high quality synthetic and you shouldn't ever have issues.
328is is no more