It's been a while, but when I had my 2000 GT, I remember arguing with the Jiffy Lube tech when he told me I was short 1 quart; he wanted to put in 6, but when I bought the oil, I checked the manual and it said 5.
Make sure you are not missing the footnote about adding 1 quart for a filter change. I don't know exactly how it is worded, but it does mention it. At any rate, the engine is designed to run on 6 quarts. I find it hard to believe that the oil level would be very far from the full mark on the dipstick after a 6 qt oil with filter change. May not be right on the mark, but certainly close. I think there may be a note in the owner's manual also stating to go by the dipstick full mark.
Ironically, in the June MMFF, the oil article talks about the large amount of horsepower you can pick up by running 5 quarts because you eliminate the parasitic loss from the internals dipping thru the surplus oil in the pan. They go on to say not to do this with the 4.6 modular engine because it requires so much upper oiling due to the overhead cam valvetrain design.
mine pings stock on 87 too. i have checked my car too make sure it's a factory program in there.. the pinging on mine is more noticeable after i upshift at around 4500 rpm's or so. i used to run 87 all the time untill i noticed this.