"Although a surrogate may rely on oral statements made by the incompetent, while competent, to exercise the incompetent's wishes to forego life-sustaining treatment, the presumption of clear and convincing evidence that attaches to a written declaration does not attach to purely oral declarations. Oral evidence, considered alone, may constitute clear and convincing evidence. However, the surrogate would bear the burden of proof if a decision based on purely oral evidence is challenged".