Radiator Leaks

I have noticed low coolant levels. Local oil change place noticed small leak on side of radiator and to keep check. This was about 5 months ago. Two weeks past I checked fluid and and was little over a gallon low. I decided it may be time to replace radiator. After the replacement and driving the overflow and radiator were still low. I have checked oil and it looks dark chocolate color. I have no milky look at oil fill and opened drain plug and small amount of liquid did appear. What would be next step? Check compression or oil cooler?