Engine oil on the filter works fine for me. I don't clean after every ride but also don't mess around on MX tracks and don't ride in other peoples dust trails. 126,000km and counting and no oil consumption.
Filter oil for outboard engines of the dude who reverses into a following sea, if you occasionally submerge your bike or if you live in the Pacific NW, Scotland or NZ 
I have serviced more than enough bikes that have had the air filters lubed with engine oil and the air filters on all were practically bone dry , so the engine oil theory doesn't work with me , filter oil is not just for wet conditions but serves as an active medium in preventing dirt from going through the filter , and that is something that will happen with a dry filter .
Unfortunately Jurgen a lot of people do ride pretty technical and dusty terrain and also in other peoples dust , so the filters do take a lot of strain .
Guys , this is not meant to be a personal attack on Le Cap , I used his reply to make a point and not to badmouth him at all so please don't take this up the wrong way .
It's the same old problem: Few people read to the page in the owners manual (what's an owners manual?) where it says you must look after your air filter more often if you ride in very dusty conditions. For me that goes without even mentioning it but I admit I am somewhat mechanically minded

If I may summarise:
You have to oil your foam filter.
The absolutely very utterly and globally important point is that a foam filter does not prevent the ingestion of anything smaller than pebbles, rodents and tweetie birds if it is not oiled.
You can use different stuff. Common to whatever you use to oil your filter is that it has to somehow cling to the foam and it has to combine with dust to slimy goo, still clinging to the filter.
Your filter will only convert a certain amount of dust to slimy goo before it's either totally blocked or before the oil is literally used up - all converted to goo, the filter has dried out and lets dust through.
Another considerable danger is the foam getting blocked by the goo and then getting pulled out of shape by the air flow.
Special foam filter oils have advantages: They are sticky and thereby cling better to the foam with less tendency of draining out of the foam. They also do not turn to margarine if mixed with water. According to Dux the filters oiled with special filter oils filter longer before needing a wash.
On the downside the tackiness makes the foam filter oils more difficult to wash out of the filter.
On a long trip foam filter oil and special filter cleaner is extra stuff to haul around.