I have always used 10W-30 Mobil One synthetic on my 94' Deville Concours. I recently tried using 15W-50 Mobil One for the summer season. I did this on my 97' Seville with no problems. In fact it was noticably smoother at highway speeds during the summer months. On the Deville, I keep getting the "check oil level" message when it is not low. In the most recent situation it was only down half a quart. Am I hurting anything by running 15W-50? I assumed that the 97' and 94' Northstar engines would respond the same to the different oil weight. Has anyone else experimented with this?