What causes an engine to run lean

no hole…hasn’t been any residue on the sidewalk since my tranny fluid leak which has since been fixed. but i will def start checking where you said and get you guys a pic as per danhr
Edit: oh i’ll pull one of the first 3…threads on #4 are starting to scare me a bit since the oil was found in the 4th plug…i don’t have a donar head for my car and don’t want to ruin the head and have to get it rethreaded