Usually, oil is a status that doubles fire damage for the inflicted party, but there is a bug in the English psone version of FFT that makes useless. I hope you can guide me to fix it. That is all. I want to fix only this bug. I like the rest of the game as it is
That's done via an ASM hack. If you download the Patcher Suite from the Tools (https://ffhacktics.com/wiki/Tools) page, I believe you should be able to find a prepackaged fix for it after opening the OrgASM tool.
Though, since we're here anyway, I should probably recommend giving The Lion War (https://ffhacktics.com/smf/index.php?board=78.0) a try. It's basically the PSX version with extra PSP content added in, and then some further additions after beating the game - the script and existing job/skill requirements are untouched. It's so vanilla, in fact, that you'd have to include the Oil fix there too!