The most common cause of oil in the coolant is a problem with the head gasket which allows higher-pressure oil to flow from an oil passage into the coolant. This may be due to the gasket itself or a warped cylinder head.
Less common, but also possible, are a cracked cylinder head or a cracked engine block.
Another possible cause, which you have ruled out with your manual transmission, is a leak in the transmission oil cooler which is usually inside the radiator. Of course this is not engine oil but transmission fluid but it can be difficult to tell the difference when mixed into the coolant.
I'd say your next step is to remove the cylinder head and inspect the head gasket for cracks or splits in the areas between oil and coolant passages.
Some vehicles also have an oil-to-water heat exchanger, often near the oil filter, and this may also be the cause if your vehicle has one.