The most important things are:

Foam density: we have tested many filters and the foam density if too large will allow dirt to get through the "cavities" in the foam no matter how good your oil is.

And the next thing is use a good tacky air filter oil. In my opinion the oil must be tacky. The regular "oil" type that is not tacky doesnt catch as much dirt passing thru the filter.

Im sure there are other good tight density filters out there but we use Origional Losi 8ight filters. And have had great luck.

That filter in the link above, if you look at the foam looks like the density is NOT very high. LArge ,coarse cavities.

Some times the filters look cool but dont actually work very well.

We have seen this many times on engines that have come in an have worn out prematurely due to ingesting dirt. The owners always kept the filter oiled and clean and could understand how dirt got in.... Thats why, filter and/or poor oil performance. Thats why we came out with filter oil and made it very tacky.

Lance
RcRenew.com