O2 sensor low voltage
system lean

explain these symptoms to me. O2 sensor was replaced ~5 months ago. Found oil concentrating at wiring point of the old o2 wires and the new o2 sensor; probably was from not changing the oil plug washer when i changed the oil. I thought that was the problem and cleaned it out real good but did not work and light came back on. About to replace O2 sensor under warranty by Autozone. Any thoughts????