Has anybody been using Iridium spark plugs in the wetheads? I spoke to the NGK technical rep, he informed me that the Iridium equivalent (NGK LMAR8AI-8) would effectively replace the standard plug (NGK LMAR8D-J), give longer life and be slightly better for performance and economy. I am interested in hearing any comments. BMW's recommendation to change plugs every 6 thousand miles seems like overkill.