what I have been using is 5GAL DSS-700 BLUE OAKFLO METALWORKING FLUID from MSC and the dilution rate is 5% (1:20).
I use it for Aluminum and Steel and it does a fine job. I have not run a bubbler in my catch tank (5 gal), and about once a year, I just dump it and start over again. Here in the humid south, I have more of a problem with evaporation and have to add water as it does "get a little thick" and because my mill/drill is not in an enclosure, overspray outside of the containment system. I go through a 5 gallon bucket in about 3 years.
Oh, by the way, the 5 gallon pail is about $50! When I started, I used Rustlic, but the price was way to much as far as I am concerned.
As far as a filter, I use some "fiberglass screen" material and it seems to do the job. Any fines that I have pass through the pump and the nozzle without clogging.