Air carrier ramp loading personnel reported a '5 gallon bucket of oil' was improperly accepted for shipment on a passenger aircraft. Ground loading agent refused to load it and referred it to the Supervisor who agreed not to load.

Narrative

While loading mail onto Aircraft X to ZZZ at XA40 there was a 5-gallon bucket with liquid inside. I placed it on the side of belt loader to look at later. I called the CSM (Customer Service Manager) on duty out to look at it. I did not load this bucket.I went to pick up the bucket and realized it was full of liquid so I set it aside to figure out what it was a bit later. I called the CSM out to the plane and he was reading the label as I was loading the other mail. He read on the shipping label that it says it is oil in the bucket.Company Y shipped with the post office a 5 gallon bucket full of marine oil. The post office sent a bucket full of marine oil as mail to be place on a passenger airplane. Company Z (our cargo/mail contract company) loaded the bucket into a cart and brought it down planeside to be loaded.I notified CSM. After flt pushed I brought the bucket into operations so that they could give it back to Company Z to return to mail facility.Company Y needs to be told that if they ship this mail it can not go onto passenger aircraft. Mail facility that accepted it needed to handle it properly and NOT load anywhere that would get it to be sent on a passenger aircraft. Cargo/mail handlers need to be trained and aware of this type of shipment so it doesn't make it planeside on a passenger aircraft.