I wouldn't be worried about the response from Castrol. They are just protecting themselves.
In the *highly unlikely* scenario where they say go 10k miles, but the manufacturer says 5k, if you did have an engine failure Between 5k and 10k theoretically the manufacturer could refuse to cover it and you could go after Castrol. Extremely unlikely but liability is liability.
I would just change it at the manufacturer stated interval. But you have the option to run an oil analysis if you like and get some data on if you can go longer.
In the *highly unlikely* scenario where they say go 10k miles, but the manufacturer says 5k, if you did have an engine failure Between 5k and 10k theoretically the manufacturer could refuse to cover it and you could go after Castrol. Extremely unlikely but liability is liability.
I would just change it at the manufacturer stated interval. But you have the option to run an oil analysis if you like and get some data on if you can go longer.