What type oil should I use???????????????

[CivicSier]

I currently have 1 day on honda oem filter with castrol GTX regular oil.
I think the previous owner had synthetic, because after changing the oil it sounded a bit louder.
Could this be all in my head? Would it be alright to switch dino to synthetic and vise versa?
What type(syn or dino) and weight would you recommend, also what brand?
What happens when synthetic mixes with regular oil if this occurred in my engine?

And what should i use valvoline, castrol, or pennzoil