Q: Oil and water mixing in expansion tank

This is a sign of a blown head gasket. When your car overheats and causes the cylinder head gasket to become compromised, this can cause a leak at the combustion chamber (which is also an area where coolant and oil passages run through your car’s engine). When this happens, this will cause the oil to leak into coolant passages and water to leak into the oil. Eventually, this will cause the car to not run at all and should be looked at by a qualified mechanic such as one from YourMechanic.