[ctesla]

so going from synthetic to conventional is not going to harm anything.. but--
1. if you used all 5qts of oil, and a pint or quart of additive, the motor is over full..
the 4 cyl. needs just 4qts, and the 6cyl. uses 4-1/2 qts total; and this is with pulling the old filter which will a 1/4-1/2 qt inside of it..
2. switch from synthetic to petroleum, and staying with 5W20 is what the accord requires, but synthetic oil can last 5k/10k/15k depending upon brand and blend, but an oil filter; even a superior one (Mobil 1, Wix, Napa Gold, etc.) is designed for about 5-7K tops, so leaving an older, used one on after an oil swap could cause a clog, an undo pressure (especially if anti-drain valve got stuck open or closed), or the paper pleats within the filter gummed up or ripped and is either hindering oil flow or cavitating..
3. if you did the oil swap without 5W20, the oil viscosity and/or fill level could cause ticking if too thin, or thick, as newer imports (and others) have much stricter tolerances on their oil pumps, so a ticking and 'whooshing' (whether due to cavitation (excess air/pressure passing thru) or thinner/thicker oil sticking or passing by) could occur; PLUS would compound noise if overly low, or overfilled..
hope this helps.