When your intake manifold gasket goes bad it can cause coolant to leak into your oil if the intake on your car seals the coolant and oil passages from each other.
Another thing that can happen with a bad intake manifold gasket is rough idling, loss of engine power, engine misfires and the car overheating.
The intake manifold gasket also seals the fuel and air mixture chambers and if too much air is leaking into the intake manifold and engine then your engine will run too rich which can increase your fuel consumption.
If you do notice coolant seeping into your oil then you likely have either a bad head gasket or most commonly a bad intake manifold gasket.
I would bet on the intake manifold gasket being bad.