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most likely the head gasket, but can be the engine oil cooler (if fitted). Is there any sludge in the sump / on the dipstick ?
One option to flush out the coolant system is to drain the current oily sludge, fill back up with tap water and add half a cup of Ariel washing powder to the coolant. Run up to temperature and drain the liquid, repeat until the system is clean.
Obviously you still need to fix the leak, but the above method does work to remove the sludge, plus your car also smells fresh for a few days lol
One option to flush out the coolant system is to drain the current oily sludge, fill back up with tap water and add half a cup of Ariel washing powder to the coolant. Run up to temperature and drain the liquid, repeat until the system is clean.
Obviously you still need to fix the leak, but the above method does work to remove the sludge, plus your car also smells fresh for a few days lol