I would also suggest following the manufactures recommendation. If Cummins calls for 30 micron primary, I would do that. I would also keep your Racors as parallel for the reasons already mentioned.
I don't know about the QSC engines, but on my QSB Cummins calls for 10 micron primary and the 2 micron secondary, so that is what I use. Now, if I ever run into problems I would consider adding a 30 micron spin on filter in front of the Racors, but I doubt I will have problems.
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