To be safe, drain out what you have in the tank and replace with your new mixture. What you drain out will run in your lawn mower. More than likely the different oils will mix properly, but I wouldn’t tell you to do that. What will be affected is the jetting if the two fuels have different gas and oil ratios. If you add your new mixture to the remaining fuel and then pour that mixture into a partially filled tank, the oil/gas ratio is going to be constantly changing. It’s not going to be a large change, but still a change. You’ll want to keep your jetting changes based on just the temperature and barometric changes during the day.
LAD Specialties customer / tony kart / rotax / kt100