Had a buddy who ran a shop at a small field in Arkansas and he kept everything in pen because he thought pencil looked unprofessional. Then a guy bought a 172 from him and came back two weeks later saying the logs showed a carb rebuild that never happened. Turns out a mechanic had written over an old entry in pen and the ink bled through from the page behind it. The old man was right, pencil lets you fix a mistake without making it look like you're hiding something.