Congrats on the new ride! My first Mustang was an 04 GT 5 speed and I miss the 2v sound from time to time.
The first thing that stands out to me is the coils. Motorcraft/OEM is the best thing for these cars and anything else can lead to misfire issues like you're having. These engines will idle fine but act up under load when the aftermarket coils go bad or if the wrong spark plugs are used. The oxygen sensor code can appear when the mixture is too lean or too rich, and is not necessarily a sign of bad oxygen sensors. If the code is coming up for both banks (P0171 & P0174 or P0172 & P0175, etc) that is a sign of a faulty MAF sensor. Do make sure it stays plugged in and if you are still having issues, try cleaning the MAF sensor. You may need to replace it.
The codes can also be from vacuum leaks and these engines have a lot of vacuum lines. There are 4 or 5 attached to the throttle body and they draw air from the EGR valve, EGR solenoid, and PCV valve. There are more lines that connect from the intake manifold to the gas tank and somewhere in between there is the EVAP purge solenoid which can also give you codes when it goes bad. Check to make sure all connections are good and if any lines are cracked or brittle it's time to replace them.
Also with the mileage getting up there, it may be time to replace the timing chain & tensioners and possibly the lash adjusters as well as the cam followers. If they've been replaced at least once in the past you don't have to rush but they may be going bad soon so I would start saving up for those.
For oil, I would recommend Motorcraft or Pennzoil 5w-20 and an FL820S filter. There aren't many substitutes for Motorcraft oil filters. Some people will disagree on the oil viscosity but the goal is to run the thinnest oil that maintains hot idle oil pressure of 15-20 psi. If it registers less pressure than that, you may have worn rod bearings among other things which isn't unreasonable on an older engine.
Thanks man! I've enjoyed it these past 2 days lol.
And the acel coils say that they do not throw cel codes specifically on the description. Plus they're $200 so they should be pretty good if not better right? And the car idles great and there's literally no problems under load either. I can accelerate to 100 with ease. No jerking , chugging, or choking. I really had no idea it was even misfiring until I got the codes read. And there was no oxygen sensor code. The person I bought it from said that was the only code coming up but when I got it read, there was 4 different codes and nothing to do with the oxygen sensor.
And okay I will clear the codes and see if it comes back up again since I plugged it back in.
As for the vaccum lines I'll be going over them when I get off work thoroughly to makes sure they're not getting worn.
And yes I'll have to start saving for it then. I probably won't mess with any timing or any of that until it's actually a problem since it's so costly and takes a while to do.
And thanks for the info about the oil. It says 5-20 on the cap I was just making sure since the mileage is high. Would it be worth switching to a synthetic or is that not a good option? Also could you tell me how to check oil pressure? Thanks again for everything you wrote up for me man!