DND - if you decide to overseed, wait until mid-Aug to mid-Sep. Cool season grasses (what we normally see in the midwest) can't establish roots fast enough to survive the hot, sometimes-dry summers. I'm not a fan of Milorganite (who knows what's in human waste?) but, if you can find it, Ringers Lawn Restore is a good one to use. I wonder how compact the clay was when they put down the sod and if it was dry, rock-hard.
