The 4.6L 2 valve engine in your grand marquis is VERY easy on oil. Your metals are a bit out of spec because of the high silicon. When silicon is high, and iron follows that means there is an air filter issue. I would bet the airbox is warped and only WIX and MOTORCRAFT fitlers fit well. Napa Gold is a wix by the way. If you visit crownvic.net look for my warped airbox fix under the body of knowledge and inspect all other sources for a leak.
Also you could be cleaning up the engine causing things to come loose, therefor skewing your UOA. Either way rest easy knowing that the 1999 model year is one of the best and that the 4.6L is near bulletproof. Your UOA is good overall.