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Can you see the oil from different angles/ distances at all? Minecraft hides entities that are off the screen, even partly, so I'm guessing that's what's happening.
Ours have done that before as well. It's mostly graphical unless you fill the tank up and then try to input more liquid. Then it causes some issues, because the whole tank glitches out and leaves the liquid texture standing. Servers tend to have a hard time with that part of it.