The F-91W is such a fun little watch[1] and people have done the weirdest stuff with it. There's a guide to make the mod on Youtube[3]. There's also the TOTP in a F-91W[2]
FWIW, a correction is required here. Oil-modding a F91W will NOT make it a dive watch replacement as the video creator claims several times.
A dive watch is designed to be operated at that depth while the weak o-rings around the pushers on the F91W will give up when you use the buttons. Oil modding on it holds up during a dive as long as you'll never push the buttons, so it's more of a neat party trick for show than anything to daily drive.