So I changed my oil on my 2000 Honda Civic recently. New filter, new oil. Drove It on the interstate perhaps 10 miles and check engine light comes on. Got both of the above codes. P-9 is a cylinder misfire I believe, and p-6 is a fuel emissions leakage. I was wondering whether the two in conjunction might suggest a root issue. If not, their separate, as it's worth noting I've been missing my gas cap for a few weeks now. The engine light didn't come on til just now however. If probably unrelated, anyone have any idea how I screwed up changing oil enough to cause a misfire? Thank you! (Additional symptoms: oil smell while driving after awhile, vibrating.)

 

The 1456 is the fuel filler cap not installed.

The 1399 is a random misfire. Probably the source of your 'vibration'. I am currently investigating one of those myself. If there are no cylinder misfires (P0301, P0302, P0303, or P0304), then it is something related to all cylinders. The list of possible causes is long:
. Fuel pump insufficient fuel pressure or amount of flow
. Fuel line clogging blockage leakage
. Fuel filter clogging
. Fuel pressure regulator stuck open
. EGR system malfunction
. Distributor malfunction
. ignition coil wire open, leakage
. ignition control module malfunction
. MAP sensor range/performance or response
. Valves carbon deposit
. Compression low
. IAC valve malfunction
. VTEC system malfunction
. Fuel does not meet Owner's Manual spec, lack of fuel
. HO2S
. HO2S circuit

Your guess is as good as mine as to what an oil change might have affected. OR They might be unrelated.