What would cause oil to leak into the coolant system on 1995 Volkswagen Jetta

I have flushed my jetta twice and there still oil in the coolant system. With that I have done thermostat water pump and a few hoses..many people re saying its either the freeze plugs, head gasket, or valve cover gasket

Asked by for the 1995 Volkswagen Jetta
1 answer
What kind of oil is mixing with the coolant,Is it engine oil than u have to check the engine oil cooler and if its transmission oil u have to check the radiator.