What are common causes for oil in the coolant reservoir in an Audi 80?

I recently noticed that there is oil mixed in with the coolant in my Audi 80's reservoir. What could be causing this issue and how can I fix it?

Oil in the coolant reservoir can be caused by a blown head gasket or a cracked engine block. A thorough inspection should be done to determine the exact cause.

If the coolant and engine oil have not been changed regularly, it can lead to oil contamination in the coolant. Make sure to stick to the recommended maintenance schedule and use high-quality fluids.