I have three SAE papers from various eras that list the maximum efficiency of the average oil bath air filter at 80-85%. One from the 1920s touted 50% efficiency!Some of the cyclonic filters in tractors (HUGE) got over 90 percent. At the time oil bath filters were replaced industrywide, cellulose were upwards of 95% efficient. Contrast both of those with the bottom dollar minimum standard of today, which is around 97% and many filters do much better... up to 99%. When you factor in how much air an engine ingests over many thousands of miles and then you pile up how much the filter caught next to how much got thru to the engine, even a difference from 97 to 99 percent makes a noticeable pile. Consider the difference between 85% and 97%! I've seen those piles in the test labs of Fram and K&N and they are part of what gave me the "gospel" about filter efficiency. If it wasn't such a PITA to post images here, I have a couple of pile shots.
The other thing about oil baths, if you let the oil get down just a few millimeters, efficiency drops like a rock. Let it get muddy... efficiency drops like a rock. That's why they recommended DAILY air filter service in some cases. The harder you run the engine, the faster the oil level drops. Oil bath filters was one reason it was nearly a miracle for an engine to last 100K miles back in the day when your truck was built. On top of all that, they don't flow for [censored]!
Years ago, when I was writing for a Land Rover magazine (LRO), I took a bunch of carbs, filters and such to the K&N lab to be airflow tested on their flowbench. That batch of stuff included a Land Rover oil bath air filter. I was shocked (but not surprised) to see how much restriction it delivered. It literally didn't have the airflow needed to allow the engine to generate rated power. That says something about the old gross (no filter) vs modern net (with filter) ratings, but I digress. We discovered that by dropping the oil level, airflow increased, but efficiency dropped. Anyway, that explaned why a Land Rover 2.25L got a noticeable boost in mid range and upper end power by installing a modern filter.
What does the once in a several-hundred-thousand years eruption of a volcano have to do with day to day use?
For a restored collector rig, obviously the oil bath is there to stay and if it's operated sparingly in a good environment, a change is uneccesary. You will be dead of old age long before wear becomes an issue. If yours is something like a '47 WDX Power Wagon that hits the dirt regularly, adapting a paper air cleaner is a good idea. I did so on my late/great M-37 Dodge, which I wheeled the poop out of. You could build a filter that didn't alter the basically stock engine and for shows, install the oil bath for the period look. But a couple thousand miles a year on nice days around town, a typical collector rig, not worth the effort IMO.