The LH cars have several tranny and engine oil cooler lines that have
clamped-rubber-hose-to-metal-line joints right at the radiator (driver's
side) that are famous for leaking (almost guaranteed to) after a couple
of years. They are hard to get to to tighten, but may be the source of
your leaking. Any oil/ATF that leaks gets blown back to cover the
underside of the oil pan, etc. Not sure that that is the source of what
you're observing, and I know you said that it is related to oil changes,
but worth considering/checking.
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