If you want to go aftermarket I like the Wix, Donaldson or Baldwin filters. The factory filter is approx a 25 micron, "absolute" according to Joe J. at Westerbeke, and the Kubota, according to my local dealer, is a 24 micron (but I have never heard of a 24 mic paper).
That being said we have no clue what the filtration efficiency is for the factory filters so that 25 micron filter may be at 98%, 99% etc. and would possibly be a 10 micron filter at 50%.... So, it still could be a 10 micron rated filter if you look at different efficiencies.? I do know that Westerbeke on engine filters are 15-17 micron rated and Westerbeke actually publishes this..
If you want to take advantage of multi-step sequential filtering, the primary taking some load and the secondary taking some load, a 30 mic on the primary and the good aftermarket or factory is a fine set up. If you have clean fuel a 10 mic on your primary will be fine and your spin on will catch virtually nothing as it is may already be filtered out beyond that spec. Parker / Racor filters are rated nominally at 95% efficiency so the 10 micron is theoretically capturing 95% of the particles at or bigger than 10 micron.. They consider "absolute" 98.6%-98.7%..
Universal advises against anything smaller than 10 mic for the spin on, though I don't believe you could even find one and of course we don't know if this is absolute or nominal..
Kubota apparently designed this product to run on 25 micron "absolute" filters, but at what efficiency we do not know, so over filtering to 2 micron is not really all that necessary.