I used to work for GM at a pontiac dealership and when I got this letter, I was kinda upset, because the letter clearly shows they really don't know what is causing it. However from my experience working on all types of 3800's, I don't believe it is from oil at the valve cover. I have really never seen extreme leaks at the valve cover and I have worked on plenty of these cars.
I have seen where fuel sits on top of the supercharger when the fuel lines leak though. And that would be cause for fire way more than the oil on the headers.
Check this.... I went to the Junkyard Monday and say 4 Grandprix's. None were GTP's. 2 GT's and 2 SE's....Tell me why both the GT's engines were burt to a crisp. BOTH OF THEM AT THE SAME JUNKYARD?!! And they say the NA's have nothing to worry about.