Silly question...
If one is _engine oil_ and the other is _hydraulic oil_, couldn't someone have decided that we'll call them... let's say:-
HFX. (X being whatever number you want), for the hydraulic fluid.
EOX, (See above for the value of X), for the engine oil...
Then, with the benefit of experience and the understanding that when a person is under pressure, (intentional pun), he doesn't need to know that OMX is hydraulic and OMY is engine... It'll be self explanatory... Despite the alarming similarity in the shape of the container - because there's a limited number of shapes...
Sorry... my mistake, that would be too easy, wouldn't it????