JaeZ33

I tried to do some research on these forums to have a checklist of what to do:

things i've done.
1) removed the ignition coils. a few of them had a little bit of oil, nothing drench or soaked. i tested their resistance with a digital multi meter and they read around 1.6-1.8. #2 look like it was replaced as the other 5 looked of the same brand.
2) removed the spark plugs, more than one had oil on it. i tried to check previous owner's maintenance records, spark plugs were replaced at 60k, car has 195k now. i am unsure if they were replaced since. the tips were kinda chalky, so i am already guessing worse outcome, worn valve/piston rings, etc. i've also read it could require spark plug seal and valve cover gasket replacement, but i wanted to do more troubleshooting before diving in the deep end.
3) i've read also you can test coil packs while engine is running. i unplugged #1, 3 and 5 and all changed idle. i also unplugged the MAF which change idle too.
4) i soaked up the oil in the wells as best i could, wiped down the plugs and the coils and put everything back in, cleared codes and started the car and waited then i got a P0306.
5) i swapped #2 coil pack to #6 and tried again, the P0306 went away and now i got a P0300 again.i also made sure #4 and #6 wires weren't swapped.

one thing i noticed. the i used my obd2 scanner and checked the sensors while the engine was running before getting the p0306: here is a link to that clip: https://photos.app.goo.gl/pqjVhfPx3yEuMDQc9
i dont know how to read everything, but i noticed the oxygen sensor 2 bank 1 and 2 are 0.28V, after i got the p0300 again bank 1 was 0.26V and bank 2 was 0.84v. Does this indicate something serious?

Can you test the ignition coil, with the intake tube removed and that bottom hose disconnected form it so i have access to the coil plugs or will it stall since there is a big vacuum leak without the intake tube installed?
Should i try swapping the #6 spark plug out with #2 and see if that changes anything at this point?
Does changing all the plugs and coils make a difference after the troubleshooting i've done so far? the coil packs should be good if they are reading OK on the digital multi meter right?