He doesn't have grounds to fire you. The most that "disobeying" his order to use conventional oil vs synthetic oii should be a warning. This doesn't sound like a case of insubordination or anything, especially if its a first time thing. If you could speak to a higher up to defend your case, this acting manager has no grounds to write you up or to fire you. If need be, call BC Employment Standards. I'm sure if you got in trouble with this acting manager, you can defend your case, especially if you know what you're doing in the job.
Good luck
