My car misfires every few weeks - why does one spark plug always get oil on it?

Every few weeks I have to take the same spark plug out to clean oil off of it because it starts misfiring. Could you please tell me what the problem may be?

When is a good time to change spark plugs?

I have a Fiesta Active X with 36k on the clock and have had it since 2021. I'm having a full service now. Does it need plugs renewed even if it starts perfectly? What's your advice?

Do I really need new spark plugs and fuel filter at my next service?

My 2019 Honda CR-V EX is due a service. It's done 53k miles, being serviced regularly. The franchised dealer recommends replacing the fuel filter and the spark plugs. I've used Shell V-Power fuel since...