Hello All
Longtime Lurker posting. 2005 G35 coupe 6MT with 88k miles purchased in 2013 with 50k, only major service clutch replacement. Burns oil usual for our cars.

My problem is a bit layered please bare with me.
The start of it. Gas gauge recently became unreliable. I went to start car parked on a slight incline. Would not start. Car read 1/4 tank. I got 2 gl and put in car.
Later the same day. Im waiting for my buddy at his home garage and that morning he offered me gas as he keeps a jersey can full on hand. He also had approx 1 gal in 2 g container. Which I put in my car waiting for him. Skip the minutia and to the point it was contaminated of unknown substance. I drove 5 blocks the engine misfiring badly and turned around. I siphoned out both sides of tank 6 gals of gas. Dried both tanks and put in 5 gal fresh and 1/.4 bottle of sea foam. Car is still misfiring. Code 300. Changed oil, spark plugs, replaced the valve cover as its leaking oil into spark 5. Added new gaskets and cleaned plenum, throttle.body, KN Typhoon intake system and MAF sensor. Still misfiring with 300 general misfire code.
Im going to do a smoke check today. Can someone please advise other steps to problem solve this? Could an ignition boot be bad?
appreciate any help
thank you