Greetings Jonaz, if it is motor oil there are two possibilities of mixing the oil with the water, the first in the cylinder head and the second in the oil cooler, it is usually at the base where the oil filter screws, you can try to make a bypass to the connection of water that passes there, leave the cooler discovered, if there is a fug in there, you will leave and you will be sure that it is sure to be the failure. With respect to the cylinder head that these are high compression engines and requires being torquet (tightening) correctly, a fault in the torque can cause compression and mixture or loss of fluids that pass through it (water and oil). If you performed the bypass as commented above and there was no loss of oil or water definitely it has to be the cylinder head packaging. Comment what happens, luck.