Water filters are 'nominally rated', which means that they let particles much larger than their micron rating through. It is a much better idea to use a filter with an absolute rating at least as low as your fuel filter. I use FUEL filters to clean my WVO. Don't bother with water block filters as they cannot/will not remove water from WVO.
Straining the oil thru sheets, tee shirts or blue jeans before the fuel filter can be an economical way to make fuel filters last longer though.
Allowing WVO to go thru the same on-board filter and contaminating the diesel tank is a proven recipe for problems. I know here on the www people claim great success, but I have yet to meet many people in the real world who are successful for long. I do not know anyone who has met more VO'ers than I have....
The diesel mazda/ranger is a sweeeet engine!
I've always said 'Choose wisely, or please post pics of the carnage!'