My DI needs some help
 

I have a DI with 138100 on it, this morning i changed the oil with synth 10W-30, new oil filter (the one that is twice the regular size). While I was letting the oil drain I started checking the spark plugs because I have been having a continuing problem with a left bank misfire (code 40), I pulled out all the plugs and they ALL have huge carbon build ups (totally black, had to scrape with a wire brush) and also found that the #6 is somewhat cross threaded. using bosch 4+ and STI wires
My question is this: what should I do next to get the misfire issue under control, plugs wires and cap are new. am I running to rich? O2 sensors need replaced? induction clean? sea foam? am using only premium and a few tanks ago used STP injector cleaner.
anything i can do about the cross thread?
also how do you clear a code on an OBD I truck?

the code clearing is easy, under the passenger kick panel there is a white and orange wire connetor, turn the key up to the second click, unplug the connector for 5 sec. plug it back in , turn off key , wait for the main relay to click under the dash and you're done.

start with a tune up.