I pulled this off wikipedia, kind of interesting and explains why they are still used.
"Oil bath air cleaners were very widely used in automotive and small engine applications until the widespread industry adoption of the paper filter in the early 1960s. Such cleaners are still used in off-road equipment where very high levels of dust are encountered, for oil bath air cleaners can sequester a great deal of dirt relative to their overall size without loss of filtration efficiency or airflow. However, the liquid oil makes cleaning and servicing such air cleaners messy and inconvenient, they must be relatively large to avoid excessive restriction at high airflow rates, and they tend to increase exhaust emissions of unburned hydrocarbons due to oil aspiration when used on spark-ignition engines".
Don't ya love it when Aldous Huxley proves to be correct one more time? "Send me a man who reads".
Look, I might be missing something, Lord knows that happens often enuf, but, the oil bath keeps coming up the winner every which way I turn this comparison.
The OB is cheaper in the long run if filter elements cost a hundred bucks.
The oil bath has no replacement materials except the oil being replaced. Any light weight oil will do....used 20wt or ATF...doesn't matter. Motor Earl is motor Earl in this case. Filters cost 100 to 125 dollars per change. How many changes do you have to make before you hit the break even point of a OB? After that it is all savings. And those savings don't end.
Fuel savings will be significant over the life of the engine.
The dry filter element traps the dirt "in series" with the air flow. That is to say that the air charge must travel "past" the dirt to get into the engine. All those tiny air passages in the element get filled by dirt as time goes on. That's how the filter works. With the filling of those holes come resistance to flow and that adds up to loss of fuel efficiency. The OB stores the dirt it traps in the bottom of the oil bath reservoir where the charge air never goes. Instead, the air going into the engine "slides" over that "slick" oil surface on its way.... (little humor there). The OB works just like the day it was borne even when it is half full of muck and dirt and twigs and talc. You keep the oil level where it should be and it works like nothing has happened even after years of deferred maint abuse. Obviously the thing can't work after the dirt gets to the oil fill line but till then all is hunky dory.
OB has more air resistance to air flow than a dry element.
I'll bet that the brand and expense of that element has a lot to do with that number. Still, even without numbers, I would say that that must be true. Here's the rub. Like I said, the resistance to air flow is going up from the time it is put into use, regarding the dry element. That should be obviously true to everyone. The resistance number for the OB must be a point that the dry element passes on it way to needing replaced. My unsupported argument is that the losses associated with the OB being less efficient at the outset are overcome by the excessive losses of the DE during its final phase of use.
Remember all the discussion about bends and length of run associated with intake air flow resistance? Who could forget? Well, that OB has a 180 degree turn in the air flow path built into it as a operating principle. That makes a lot of resistance but the DE is restrictive by its nature, as well.
OB is more labor intensive and costly than a DE
The DE needs to be replaced on a "time run" and "conditions" basis. The OB needs "checked" on the same basis. Actual servicing intervals are much much less frequent. Operating in Southern California I got 1/8 inch of "clay" out of my OB filter after one year and 20K miles. I didn't clean it again for 4 years but I checked it every 6 months....religiously. Elements were only a few bucks back then.
CON: The OB works on the centrifugal forces associated with an air stream abruptly changing direction. The 180 degree direction change in the bottom of the filter "flings" the dirt particles "at" the surface of the oil, where it sticks and sinks. Remaining air flow is "clean". The finer and lighter the dirt particles the faster the air must be traveling to be thrown into the oil. "Smoke" would not likely be trapped....also doesn't do much damage due to abrasion. BUT there is a trade off: the faster you make the air flow the better the filtering but the greater the air resistance and subsequent negative impact on economy. The OB in autom otive use is a trade off. To filter the air must travel fast and the design is to get acceptable filtering at the most used air velocity thru-put. That would be cruise and the desine would be pecular to the engine displacement. Move the RPM up to MAX and the OB is sucking harder and not as free flowing. At idle the velocity of the air at the turn isn't great enuf to allow the best filtering. Turns out, as my guess, there is more than one reason for DD being adament about not letting the engine "idle" for very long at all. Air filtering, it seems, on those "olde timey" 2 strokes got poor at idle. Sand and grit will not make the velocity turn regardless and less of that can be moved at idle but the talc is always there.
Here's the main point: They still make the oil bath filter. There is a market for them. I am told that all rock quarry equip is equipped with the OB filtration and that the rock crusher running at the bottom of the pit is most certainly OB equipped. That comes from a D mechanic that worked in more than one quarry. Farm equip, still comes with OB filters in some cases and here again....I been told. My call to Caterpillar got me the following: They are no longer standard equip on OTR truck engines. Until recently they were an option. That rep said he knew that 3408 engines for trucks came with the option of a OB filter. His "guess" was that heavy equip still used them but where DE filters used to be the option the OB was now the option. Times change and not always for the better....mostly, but not always.
Today, the mfr gets a profit on DE replacement parts. Not everyone sold but the dealers still install their house brands and, while they are made by suppliers, they are usually way more expensive. They get not a nickel for OB after it leaves the factory. True, they get more to service the OB but most service is not done at the dealer. Lots of crossed motives and reasons of profit in this mix.
I would choose the OB if it were an option. I would certainly install a "Filter Minder" on every DE equipped engine....as I have done. And I would change no filter before it's time nor run it beyond it time.