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Engine oil wicking along a wiring harness into a connector, usually from a failed sensor or actuator seal, and the reason it is worth naming separately from water ingress is that it acts differently. Water corrodes and eventually conducts; oil does neither. It insulates, so the failure is a connection that is present but no longer reliable.

The route is the surprising part. Oil enters where a component's seal has failed, then travels along the conductor inside the insulation by capillary action — so it can reach a connector well away from the leak, and can even reach the module at the far end. A connector full of oil often has a dry-looking component at the other end of the harness, which is why the obvious inspection point can look innocent.

Rocker arm and valve lift actuators are where this corpus meets it most, since they are threaded into oil galleries and their seals live in hot pressurised oil. Cleaning the connector is only half the repair: if the seal that let the oil in is not replaced, the joint fills again. And check the module end before assuming the damage is local — oil that has travelled the whole harness will have reached a connector nobody thought to open.