Hi George,
I'd expect that, when marketing a filter for use as an Nth stage fuel filter, the advertised "micron rating" would correspond to something in the 95% - 98% efficiency range. So, I'd think of your filters as being comparable to others sold as "20 micron (nominal)" filters.
If someone wanted to call them 15 micron filters, I think there is still a solid strong basis for that. In my mind, 10 microns would be a big stretch. And, given the application, labeling them with a smaller number than that would be a off the deep end. I think.
Given that, unless the manufacturer of the motor had another recommendation, I'd expect to find a 30 (or even 40) micron (at 95+%) primary filter upstream. That would likely be enough to protect the primary filter from getting fouled too quickly without changing anything else.
In my thinking, running with a 15 micron or 20 micron filter upstream approximately makes the primary filter into the secondary filter and the secondary filter into a back-up. The filtering would still be to 15 or 20 microns to 95+% efficiency. And, that is fine. your engine would essentially get just as clean fuel as with a 30 micron filter -- but you'd end up having to mess with the primary fuel filter only rarely. The primary would get most of the chunks out.
With a better micron rating than that, e.g. 10 microns, 5 microns, 2 microns (at 95+%), you'd be feeding cleaner fuel to your engine that it otherwise would see. And, you'd be using the 2ndary filter very little -- the primary filter would be much finer.
2 and 5 micron filters just trap so much that probably isn't harmful to old-style engines that I wouldn't bother. It'll just load a filter sooner than need be, forcing more maintenance and corresponding down time than need happen. I think.
So, my choice for a primary would be in the 10-30 micron range. And, so I'd go 10 micron, unless a 30 micron happened to be in the locker or on a super cheap sale, in which case I, personally wouldn't hesitate to use a 30 micron filter.
Just me thinking aloud.
Cheers!
-Greg