There is only one possibility of water ingress into the diesel filter element, and it is impossible for water to be generated in the fuel tank out of thin air due to external entry. It's not a matter of oil quality.
A filter refers to an accessory that filters impurities or gases through filter paper. Generally, it refers to a car filter, which is an accessory of the engine.
According to different filtering functions, it is divided into: oil filter, fuel filter (gasoline filter, diesel filter, oil-water separator, hydraulic filter), air filter, air conditioning filter, etc.
There are three types of filters for engines: air, oil, and fuel, commonly known as the "three filters". In addition, an air conditioning filter is commonly known as the "four filters". Respectively responsible for filtering the medium in the lubrication system, combustion system, engine intake system, and carriage air circulation system.

Fuel filter:
Its function is to filter out harmful particles and water in the engine fuel gas system, to protect the oil pump nozzle, cylinder liner, piston ring, etc., reduce wear, and avoid blockage. Filter out the iron oxide contained in the fuel.
Even though diesel has undergone sedimentation and filtration before being added to the diesel engine fuel tank, it is still contaminated during the refueling process due to factors such as refueling tools, refueling environment, and unclean fuel tank ports.
Moreover, during the operation of a diesel engine, the impurities deposited in the fuel system and suspended dust in the air can also cause diesel pollution. Therefore, a diesel filter on the vehicle is essential, and diesel may not be truly clean before being added to the fuel tank.