is the line that serves as overflow tanks of oil to a tank specific to prevent the oil overflows into the sea.
Most likely - broken gasket.
Usually a blown head gasket.
Someone is putting it there. There is no way for oil to get from the engine to your fuel tank by natural means. Literally, if you have oil in your fuel tank, someone is sabotaging your vehicle by putting the oil there.
Ship fuel is not fed straight into the main engine, as first of all the HFO (Heavy Fuel Oil) is pumped and then separated into a settling tank (buffer tank). The settling tank is connected to the service tank (day tank). This service tank is filled via a continuous overflow from the settling tank and is also connected with the main engine. The oil is heated in both the settling and service tank to make sure that an even temperature of between +75°C and +90°C (or +167°F and +194°F) is maintained, as this keeps the heavy fuel oil (HFO) at a consistency that can be pumped easily.
unhook the overflow hose from the fuel filler hose and put a syphon hose through it to the tank. I used a 3/8 hose.
If is oil then you have a bad intake manifold gasket. If it is transmission fluid then the cooling tank in the radiator is bad.
A gas tank can indeed overflow if it is not full. A gas tank can overflow if the pressure inside of it is too high.
Vaccuum canister, to remove fuel vapors from the gas tank.
It is to let you know when an underground oil tank is filled to its capacity so not to overflow.
Some truckers use the oil from an oil change as added fuel in the tank. In this case it will Not ruin your tank
In the fuel tank
The question is how did oil get in the fuel tank? If there was very little oil put in the tank as in less than a couple of quarts then just fill the tank full and drive the car. The oil will be burned with the fuel and will do no harm. If there was more than a couple of quarts, you can siphon it out with a hand pump. You can get enough out to then fill the tank and drive the rest out.