Most likely the oil under that plastic cover is either from spilled oil, easily spilled over the filler during an oil change because the "white engines" take oil slowly and it's easy to add oil too fast spilling it OR the fill cap seal is old and dryed out (replace it).

Do NOT pressure wash that engine unless you have the cover off and have access to air pressure with an air gun because water often fills the spark plug holes whenever those engines are pressure washed and that causes problems. If you do wash that area, remove the splug plug wires and blow the water out of those holes before trying to start the engine.

I use brakeclean instead and blow it off but you could soak it up with rags too if you don't have access to air pressure.

Before looking for more oil leak sources, degrease the areas in question (brakeclean works well for that, safer than water) and run the engine once degreased to see if you still have any other leaks.