With the last update i noticed you created your own type of oil, the funny thing is you can take this out of the tanker endlessly, when its stationary and put it back in when the tanker is empty or unloading for example

Edit: quick gif to show you what i mean https://gyazo.com/b62b98958725c4872dfb67ab5a00875d

No you can't ;) In this picture you can see the actual liquid level. The items you take out are useless (only there to make the conditions work) and Railtanker overwrites the amount with the actual liquid. Take some of the items and put them into an empty tanker, they disappear after about 1 second without adding any liquid

Ah ok makes sense :)