FE content in oil

The FE (iron) content can go up and down. This can be normal. Just keep an eye on it and make sure you do timely oil changes, good tank flushes and inspect the magnetic plug and oil filter each oil change. I do a lot of oil sampling on all my clients over a certain number of logged hours. I get copies of all the analysis.

It can also be affected by how you collect your sample. You shouldn't collect it at the begining of the drain nor at the end. Catch the sample in the middle of the oil drain. At the very begining of the drain you will usually see higher concentrations of heavy contaminants that sink to the bottom of the tank. 

This is another reason not to use Quick Drain valves. You get a very poor tank flush vs just removing the tank plug. The Quick Drain valve will allow some of those heavy contaminates to stay in the tank.