Reducing methane (CH4) is a key objective to address climate change quickly. Manure management and storage play a significant role. In this context, a real-scale trial was performed to measure the ability of the commercial additive SOP LAGOON to reduce carbon-based greenhouse gas (GHG) emissions from liquid manure over approximately 4 months. Gas emissions were measured at a commercial dairy farm from two slurry tanks, one treated with the abovementioned product (SL) and the other used as the untreated control (UNT). After 3 and 4 months from the first additive applications, the SL storage tank showed lower and statistically significantly different emissions concerning the UNT (up to −80% for CH4 and −75% for CO2, p < 0.001), confirming and showing improved results from those reported in the previous small-scale works. The pH of the UNT tank was lower than that of the SL on two dates, while the other chemical characteristics of the slurry were not affected. In this work, SOP LAGOON proved to be an effective additive to help the farmers mitigate the contribution of stored liquid manure to global CH4 emissions, potentially improving the overall sustainability of the dairy industry.