It is very important to ensure that the oil bath air filter (reusable type) filters oil evenly after cleaning, as this directly affects its filtration efficiency and the normal operation of the engine. Here are some key steps and techniques to ensure even oil filtration:

Use the correct oil filter and tools:
Specialized oil filter: It is necessary to use the manufacturer's recommended specialized filter element to filter the oil. This oil has appropriate viscosity and adhesion, can form a uniform protective layer on the surface of the filter material, and is not easily inhaled into the engine.
Suitable application tools: You can use a soft bristled brush (such as an old toothbrush or a specialized filter brush), a clean cloth, a sponge, and even some brands offer oil filters in the form of sprays. Choose tools that allow you to operate with precision.

Thoroughly and evenly apply:
Partition processing: Do not attempt to fill the entire filter cartridge at once. Divide the filter element into several areas and process them one by one.
From inside to outside/top to bottom: It is recommended to apply the inner side (windward side) of the filter element first, and then apply the outer side. Or follow a certain order (such as from top to bottom) to avoid omissions.
Apply wrinkles: This is the key! There are many folds in the air filter, so it is important to ensure that the filtered oil can penetrate into the interior and bottom of each fold. Gently brush or wipe along the folds with a brush or cloth to ensure that oil can enter.
Thin layer coverage: The goal is to form a thin and uniform oil film, rather than making the filter element greasy. Imagine spreading jam on bread, thin and even.
Moderate intensity: Apply gently to avoid damaging the filter material.

Allow the filter material to fully absorb:
Rest time: After applying the filtered oil, do not immediately proceed to the next step. Let the filter stand for a period of time (usually recommended 15-30 minutes, refer to the oil filtration instructions for details), allowing the filter material enough time to absorb the filtered oil. The absorbed filter material will appear fuller and darker in color.

Remove excess filter oil:
Wiping: After standing still, gently wipe the inner and outer surfaces of the filter element with a clean, lint free cloth (such as a microfiber cloth). The key is to remove excess filter oil that has not been absorbed by the filter material and floats on the surface.
Inspection: After wiping, check the surface of the filter element. The ideal state is to look oily but not greasy, without obvious oil droplets or stains. Gently touch the surface and you should feel oil but not stick to your hands.
Confirm wrinkles again: Check the inside of the wrinkles again to ensure that there are no omissions and that the oil inside the wrinkles has not been completely wiped off by wiping.

Uniformity inspection method:
Visual inspection: Observing the filter element from different angles, the color should be relatively uniform and there should be no obvious dry or oily areas.
Touch check: Gently touch different parts of the filter element with your fingers to feel if the oil level is consistent.
(Optional) Weight check: If you have the conditions, you can weigh them separately before cleaning, after adding filter oil, and after removing excess filter oil, which can roughly determine whether the filter oil amount is within a reasonable range (but this is usually unnecessary and requires experience judgment).

Summarize key points:
Patience and meticulousness: Do not rush to achieve results, handle tasks in different areas, and ensure that every corner is taken care of.
Thin and uniform: The goal is a uniform thin oil film, not a thick oil layer.
Fully absorbing and removing excess: This is the core step to ensure uniformity.
Using the correct tools and filtering oil: basic guarantee.
If you are not proficient in the first operation, try a few more times and you will find the feeling. If you are really worried or the filter has been used for a long time, it may be a safer choice to consider replacing it directly with a new filter.