What would cause my gear shift from working? What would cause oil to be in the spark plug cavity?

Is this a manual or auto matic? And how do you mean it won't shift? Make sure you foot is on the brake and pressing it down before attempting to shift it.

It cold be your shift interlock not relasing as well if it's an automatic.

You got two diferent problems here

1. Gear shift won' come out of Park-check brake lights if its working if okay-have shift lock solenoid checked-

2. Have the valve cover gasket and seals checked-

Just to add to this your old valve cover gasket is molded from silicone and held in place with more silicone. Over time the silcone compresses and leaks. Felpro gaskets has a runner gasket replacement. All you do is carefully remove all the old silicone and clean up the surface and replace it with the new gasket. Its for sure the gasker leaking have replaced tons of them for that.

Its in the center console below the shift lever.

If you have power windows you can remove the power window switch from the console to get to the solenoid for testing. If you dont have power windows you can remove that pocket they put in place of the window switch for testing of the circuit. Do you have a multimeter to do some testing?