What kind of fuel should my 2006 Honda civic get?

I just recently got the car. The car that I owned before was an older '97 model and I was used to getting regular(87 octane) gas for it. With this car, I got it from a used dealer/car shop and the car had little gas in it. So when I purchased the car, I had to go straight to the gas station to fill it up and I used the regular grade. The car was fine that day, but the next morning the check engine light came on. Everything was inespected, or so I've been told, from the oil being changed, brakes, transmission etc. I recently went back to the dealer so they could check the engine to see what was causing the light to be on. After the mechanic did his thing, he asked me what kind of grade of gas I was using and I told him regular which he then told me that that was the cause of the light being on and that I should use plus (89 octane) instead since its a newer model car. However, in the manual it said to use grade 87 OR UP. So basically should I listen to the manual or the mechanic? And while I'm waiting for my gas to deplete so that I can use the next grade, could anything acutely serious happen to my engine or am I fine for now and just use the next grade higher next time?

Go by the manual. 87 is fine. Very few cars actually require 89 or more. The engine light could be caused from a loose gas cap.

You may need to use a higher octane because the engine is worn. Worn motors will ping on regular so you use a higher octane to silence it.

What that mechanic told you about using high octaine because its a new car is total BS and idiotic. Octane is based on compression. Hes a snake… Blatant flat out lie.

regular gas shouldn't make the check engine light come on though. That mechanic is talking weird… Get a second opinion… That's just BS. Is this a dealership or a "Car lot"? Like one of those tote the note place? Get a mechaninc not associated with that lot to look at it.

hes screwing you around. No way would the gas make the check engine light come on. The car is designed to run on regular! Maybe he saying that to stall you past the warranty period so they don't have to fix the piece of junk they scammed you into buying.

absolutely nothing bad will happen mixing grades of gas in the gastank.

your car should get regular, unless it knocks or pings on regular, then you'd use premium or octane boost to shut it up.

He is a liar.

The manual was written by someone at the factory where the car was designed and built… They should know what fuel it needs.

Used car dealer he is just trying to make you go away take it to a compatent mechanic did you close your gas cap all the way? You can go to auto zone they will tell you what the engine light means

I highly doubt that using 87 octane gasoline would set a CEL and code. What was the code? You're getting messed with. Salesmen at car dealers are notorious for erasing codes in used vehicles so they can sell them with out the repairs coming off of their commission check. That light is going to come back on. Have the codes read at an auto store and then post the actual codes here. Don't have them erase the codes. You'll want them in the computer yet for when you make the dealer repair the vehicle for free! Make sure you accuse them of erasing the codes before selling you the vehicle. They know what's going on!