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Hello I will help you with your question,
Castrol like most oil manufacturers makes oil from conventional mineral based oils and synthetic base stocks and sometimes blends them.
If you want to run synthetic you can just read the label or notify the service writer that is what you want. The synthetic blends are good oils but they may only be 5% synthetic and the rest mineral based.
You can run conventional if you keep up the oil change interval, synthetic has little value unless you are pulling heavy loads or run in a hot climate for extended periods then it will hold its additive package better than conventional.
What you really want to see on the label is that is meets or exceeds the ILSAC-4 or GF-5 specification and is 5W-20 weight oil. Other than that you are free to use any brand you wish.
Let me know how I can help you with this