I'd like to add a small addition/correction to the above.
An oil bath air cleaner works by two methods:
First, the oil is used to entrap dirt, that is correct. But the cleaning action is 90% accomplished by the rapid change in direction the air must make as it get sucked 'down' then back 'up'.
HERE is a simple diagram The last little bit of cleaning is done by the oil coated wire mesh.. this is simply hoping that the light dirt that didn't fall out of suspension collides with the sticky oil on the wire.
Thicker oil does nothing, and can actually have a detrimental affect (due to higher surface tension not allowing the dirt to adhere to the oil... but simply bounce off it.
Paper filters are not 'better' per se, just easier to maintain (no oil to mess with, no metal mesh to clean. Also, oil bath filter flow air much much better, paper filters are designed to clog up as the dirt fills the element.
