If it continues to smoke after start up, and it's gray smoke like you said, then your oil is slipping through to the cylinder and it's more than likely rings or valve seals/guides. I don't think it's valve seals because they expand as temperature increases. This means they would block the oil from coming in unless your oil has thinned out so much that it slips past rings and seals anyway. Change your oil if you haven't done that in a while and/or check compression like someone already said.