The common types of front fork of mountain bike are: damping rubber, spring, oil spring, oil gas and double gas. Damping rubber and spring damping front fork belong to low-end front fork, oil spring belongs to middle and low end, oil and gas belongs to middle and high end, and double-lift is high-end front fork. Of course, there is also Fox's smart X, but it was specialized purchased as early as 2008, so I won't go into details here.
After the spring fork is used for a period of time, it will leave a little sludge on the pipe wall of the fork, which means that the fork will leak oil. This is also normal. After all, this thing is moving, and it will wear more or less, which will reduce the tightness. I've seen damping glue, but it's hard to say whether it's sludge.
The maintenance of fork is that ordinary non-professionals can only wipe dust and dirt, and the idea of dripping lubricating oil on the pipe wall is very incorrect. Facts also tell us that dripping lubricating oil basically has no effect. If the front fork of mountain bike is disassembled by non-violent means, it will be found that its internal structure is still relatively complicated, and the idea of dripping lubricating oil on the pipe wall is completely ruled out.
That's all I can think of for the time being I hope I can answer your question.