If I was the guy designing it, I’d have an add pack designed for 3 quarts, and something to help adjust the TBN of the existing oil. Someone has to take a bullet for the team and have a VOA done.
Keep in mind, over 15 years, this mower might only see 150 to say, 300 hours, equivalent to 9,000 to about 18,000 miles on a car plus you are replacing 25 % of the oil every 10 to 20 hours. Keep in mind this is a heck of a lot better than the Briggs and Stratton never-change-oil technique.