Your profile does not say where you are - like humidity and temperature environment. Looking at the time between changes, it appears the plane can sit a while. Depending how you fly - short vs long times, and how long the oil is hot, and if you use heaters it could be some mild corrosion due to internal moisture.
Silicon is not elevated, so it is not getting dusted.
If that is the case then changing your routine may help drop the iron. May not, but worth a try.
Do you have an oil separator or a straight exit breather?, is there any water related whiteness/condensate on the dipstick?
All these thing might give some clues, or it is a piston pin scraping the cylinder wall, or a slowly eroding cam/lifter unrelated to operation and nothing to do but open ad repair. In any case, more investigation is in order to prevent proper discovery of the root cause later on. No use in getting oil samples if no action is taken on the results. It may, indeed, be a fundamental issue unrelated to anything but chance.
PRL - what kind of barrels, and rings? Probably not ring de-lamination as oil consumption would be rising, so scratch that.
I really don't like iron in the filter after the initial oil changes cleans everything out. Good luck.
Last edited by BillL : 05-14-2017 at 10:26 AM.