Welcome to HF.

My thought is to stop blowing $$ on any other Octane than 87 and forget miracle fuel cleaners, just always use quality 87 fuel.

An easy way to see if you have a (P0305) misfire at #5 is to swap out the Coil On Plug pack #5 to a different cylinder .... say #2 etc., write that down so you can remember where it went. Clear the code and drive it another tank of quality 87 fuel. See if the CEL comes back. If it does throw a code, see if the code followed the COP #5 to its new location at #2 (P0302).

If it does not throw a code its all good.

If it throws a P0302, then you know the old COP from #5 needs to be replaced. If it still kicks a P0305, let us know, there are more things to be done. You are welcome to search what to do with a misfire code meanwhile.

More likely to be a bad/tired COP than something else. Do the easy stuff first.

Good luck.

 

 

 

 

 

P0300 is random misfire, means the knock sensor and spark input to the PCM thinks it might be detecting a misfire, but it is not consistent enough to be sure where, or it may be moving. That may actually be good, because it may be fuel injector related (your magic juice may have busted some crud loose causing and temporary irregular fuel injector pulse). That is where my comment came from originally, drive a tank of quality 87 through it and it should settle down.

I have heard of people adding fuel cleaner to older vehicles and within days/miles, the fuel pump toasted. Theory being, an old fuel tank has gunk inside stuck to the tank walls/floor ..... You add cleaner, it dissipates the gunk which then gets into the fuel pump causing it to be under additional strain pumping gunk and fuel until it dies.

 

I'd start by giving it a tune up and some new C.O.P's... Then clean MAF and TB. Possibly add a new air filter too. Seeings how you just got it and probably ready for some tlc.