98 K1500 5.7L P0305 headache

I have a 98 GMC K1500 4X4 with a 5.7l. Back in Feb I threw a SES light. It went away and the truck was running ok. I read the code and decided on a full tune up. The number 5 plug was crusted with crap and was black kinda on one side. All ACDelco plugs, wires, cap, rotor, both pre-cat O2 sensors. Ran a bottle of top end cleaner down it before the tuneup also. Clearded the codes and went for as ride. seemed ok but for awhile. Then it started up again with the same P0305. Pulled the #5 out and the same crap was building up again. This happened 3 more times then research indicated maybe the injector system was at fault. Ordered and installed the MFI replacement for the SCPI one I had. Still had a rough idle and #5 still with the same black coating on half the plug and bridging building up on the electrode. I also have an oil coating on the threads and taper when I pull the plug. Don't know if that is an indicator of anything. I am at the point of trying valve stem seal replacement. The compression appeared good and similar and I know the black pattern may be from the intake valve opening and pulling the mixture across the plug. My mechanic want to replace the intake manifold gaskets but I'm trying other paths first that I can do. Since I bought the truck I have used Mobil One and never stressed it out towing.