I have little brown oil in the water overflow and little water mixing with my oil, but te car runs like new?

That's because the oil is attacking and contaminating the rubber. Causing it to swell, soften, and become weak. The oil could be coming from the engine oil cooler in the valley, or even the transmission oil cooler in the radiator. Unless you know the condition of your transmission fluid as red, it could be brown. Since there is no dipstick for the trans, it often goes neglected.