oil is not just oil
i seriously cant believe your using castrol gtx in your 172......im hopin its the magnatec version
give a mineral oil some heat and grief and it will start to break down and give you no protection, which is why there is synthetics on the market today
another advantage, which a mineral oil does not give, is that they have a content of synthetic ester, which sticks to metal surfaces....i.e. the internals of your engine, meanin that come start up in the mornin, your internals already have a nice coating of oil so there protected straight away
castrol gtx might be ok for a top up, but dont use that as your main oil, you need a semi at least, and then id advise drainin every 6k