PIG
IMO the corrosion is as a result of moisture providing the electrolyte for dissimilar metals to create electrolytic action, the most likely scenario is the piston rings have just cleaned the cylinder of oil and the piston randomly stopped above TDC this the puts the piston & bore in the perfect place for electrolytic action to cause corrosion. At the next engine start the corrosion is removed and the process continues over a few months until the ridges at BDC start to bacon slice the piston pin plugs. This is when the evidence starts to turn up in the oil filter.
This problem is particularly common on Lycoming O-235 engines installed in C152’s, the C152 is over cooled for the U.K. environment and in the winter a combination of low usage and a moist environment sets the corrosion process on its way. The same engine installed in the tightly cowled Robin DR400 is far less prone to this problem as the engine oil temperature reaches the 80c required to evaporate the moisture on all but the shortest flights.
I won’t link to the next clickbait video instalment 
but can report that the guy sent the cylinders to [another company he did a promo deal with] and they found a slightly loose piston pin plug, which was presumably the source of the elevated aluminium. They found some cylinder wall pitting, as was seen earlier. They also found a bit of erosion of exhaust ports (this engine had a long hard time in some previous life). So they told him he needs six new cylinders, which by amazing luck (cylinder kits are short right now) they had on the shelf, qty 6.
