Wow, that is extremely scary that you are getting your oil changed for $18. You cannot buy 8.5 quarts of oil for that price, much less the filter. These engines are built to extremely tight tolerances and require 0W40 Mobil One synthetic or comparable oil. The mobil One oil runs around $7 a quart, so do the math. $63 for oil alone. Figure another $12-15 for a filter. You should get a minimum of 7.5K miles out of an oil change. On average, it will cost you $80 to do an oil change yourself. I would be worried about my engine if you are paying someone $18 to do it.

Overkill means you are changing it too often. But, in light of what you said, not only are you doing it too often, but not getting it done properly. Poor, poor engine.