Why is my 2000 mustang 3.8l coding misfire #1 cyl?

This has been coding # cyl misfire for quite some time. You can feel the misfire at idle & cruising speed with no load on engine. Now the question: I have concentrated all my efforts on #1 Cyl. Finally decided to do a compression test on all cyls. Found #6 plug completely fouled with ash buildup. Obviously #6 is the misfire. Why does it keep coding #1? Also removed upper intake manifold & found oil & carbon in both intake & exhaust ports on lower intake manifold. (some cyls. Not all). What would be the cause of this? Please help, I'm biting myself in the back trying to find out the cause. Compression is good on 3 cyls, Marginal on 2 & one is below limits.

I recommend you take the Mustang to someone (preferably a local mechanic shop) to connect a real scanner on it, not a cheap one that the auto parts store use and sell. The mechanic will have to run a "Cylinder Contribution Test" which will show how every cylinder is working in real time. Numbers in the negatives will show that particular cylinder to have an issue. If you are have excessive oil and carbon build-up in the intake, you might have a head gasket or piston ring issue. Head gasket issues on Ford 3.8 motors are a little more common. Before sinking in too much time or money into that job, get a price on a low mileage salvaged engine. You might save a lot of trouble and money by going that route. Hope I could be of help.