I found there are lot of black stains in the side wall in coolent rserver could that be oil is leaking into the coolent and got dry but I didn't see any oil foating in coolent I hope I'm not getting a blwen gasket

95se 66k miles

does it smell like gas when you open the coolant reservoir?
Does your oil cap have a white foam on the underside? If so it's a blown gasket.

When you stop the car does your reservoir bubble or gurgle?