1998 Cadillac Deville white smoke coming from tail pipes

Your oil life monitor is probably more responsible for the change oil message. You say is has a coolant leak that you haven't found, are you seeing coolant on the ground. The white smoke/steam that diminishes as it warms is indicative of a possible internal leak. If you look at some of the posts in this forum regarding overheats and coolant related issues, you would see it is fairly common. My suggestion would be a pressure test, and block check for combustion gases in the cooling system. The fact that it lessens when warm could be due to expansion, also the metal getting hot and boiling the water earlier.