Pontiac Grand AM 2000 89k miles 3.1v6

I recently had a tuneup that served as a tunedown: spark plugs + wires, fuel filter, pcv valve all replaced. It started running a little rough during breaking and idle. In addition, there was a faint fuel (oil?) smell which entered through the passenger ventilation after idling for a while.

So I took it back. It had a code 301 - misfiring cylinder 1. The diagnostics reported an occasional (<1%) misfire on some of the other cylinders, but there were no other codes. The repair shop replaced the plugs and wires. The faulty cylinder 1 seemed fixed but the rare misfires were still there.

But I have noticed that the fuel smell is still present. What is the most likely cause?