That statement about finding such a relatively large piece of gear steel debris inside the heat exchanger core seemed odd to me at first, since it would have had to pass thru the oil pump inlet screen to end up there. However, after looking at
this image of the oil pump inlet screens it does seem possible that such a relatively large piece of debris could pass thru due to the coarse mesh size.
One thing that AH might consider is using a finer mesh size for the pump inlet screens. To me, the existing inlet screen mesh seems too coarse. Obviously using a finer mesh size would require increasing the screen area to maintain acceptable flow characteristics. Logically, the size of the screen mesh should be capable of stopping any debris particles that could not pass through the pump without causing unacceptable damage to the steel pump elements. I don't believe it could be demonstrated by analysis that this would be the case with the relatively large piece of gear steel debris found in the heat exchanger.
Would be interesting to see the condition of the oil pump elements, assuming they were recovered.