Coolant in oil

1) About a year ago my engine light came on, I got it checked and the mechanic said it was something about exhaust fumes not being disposed of in the engine, therefore coming out of the exhaust. But said it was nothing to worry about and that it was only a matter of emissions. He proceeded to turn of the check engine light. The check engine light only comes on every few months, due to the same problem.

2) About a month I noticed that my engine was never fully heating up even when the car was running for considerable amount of time (during a Canadian winter). Now with warmer weather, the temperature gauge on the dash takes a prolonged time to reach operating temperature and fluctuates as I drive. I checked my coolant reservoir and it was empty. I filled it up a few days ago, checked today and it is empty. I also took out the oil dipstick and noticed that there is a brown crust, which I have read up on, occurs when oil is mixed with coolant. I thought it might have been a faulty thermostat but I do not think that explains the oil and coolant mixing.

Cheers