Donaldson and probably Parker too have many different filters available and also filters specifically for removing water. If you're starting off with new oil I think the 20 and 3 micron filters treemuncher mentioned would be good or even 20 and 5. First filter could also be a 10 micron. If you're also filtering existing oil you can always run the oil through the filter cart 2 or more times. I seem to recall the place I rented a filter cart from said that some oil may be a little thick/wrong viscosity for filtering lower than 5 microns. I think there's 4 micron filters as well. The shop I went to had their oil for the test bench go through a continuous loop 24/7 to ensure it was the absolute cleanest possible. I was surprised by that. Biggest issue is the filters cost about $100cad. each. They don't need to be changed too often if running new oil but if cleaning an existing system could get a little costly if there's a lot of contamination. Still much better than taking a chance that all the contamination is removed.