Dreaded misfire code....

[Needmoremoney]

'99 denali 5.7 vortec with 145k on it. At 115k developed severe miss, acutron codereader showed P0300. No spark to cylinder 3, replaced cap & rotor-which left me wondering how truck even ran, cleared code, & has been fine til recently. Been running bp 93 ultimate for past 15k miles, along with a bottle of techron premium & bottle of 108 octane boost every 2k miles, or less, to combat an occasional miss up long inclines. That seemed to do the trick & everything's been fine til now. Misfire returned, but not as bad as last time. Replaced cap & rotor-which were pretty bad, reset code only for it to quickly return, but now as a P0303. I changed all 8 plugs-cylinder 3 was caked over with a large black clump of grease-like gunk. & changed the wires too. Ran great-for about 5 miles, P0303 returned. I added a small bottle of techron, 1/2 bottle of 108, 'blew out the carbon' as it were, a puff of dark smoke, it ran fine for a day or so til slight misfire returned. I repeated the techron/108/jumping on it with the same shortlived result.
Sooooo....i get that there may be something requiring more work, but am looking for honest answers to the following questions-
*any adverse effects to using techron & 108 far SOONER then suggested use-say 200-300 miles rather then 3k miles?
& my BIGGER question-
*with regards to spark plugs, i used AP-605 autolites (single 'P', NOT. 'APP'). Do you REALLY think AC delco 41-993 would make THAT MUCH of a difference?
**and, would there be any adverse affect if i threw just 1 41-993 into cylinder 3, leaving the other 7 autolites? Temporarily at least?

 

[o_fatmike_o]

I have used auto lights when on a budget but yes the ACDELCO quality is superior . I cant say for sure it will clear up your issue , but they will spark better . no you could try installing just one, if it happens to work i would eventually upgrade the other 7

 

[scotto0609]

just switch the plug with #1 plug and see if misfire moves