Does anyone know if all this has anything to do with the removal of ZDDP?

ALSO, back in the early 1990s Mobil had a class action case with piston engine aircraft owners following damage linked to poor oil chemistry.

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1987 300D (230,000 mi on a #14 head-watching the temp gauge and keeping the ghost in the machine)
Raleigh NC - Home of deep fried sushi!