Wind turbine gearbox degradation driven by lubricating oil contamination represents one of the most environmentally and economically consequential challenges facing modern wind energy operations. This study proposes two optimized maintenance strategies designed to extend gear train service life, reduce hazardous waste generation, and lower the carbon footprint inherent to conventional intervention practices. The first strategy employs continuous oil contamination monitoring. When particulate concentrations exceed a predefined limit, production output is temporarily reduced to enable oil filtration, preserving lubricant integrity without full system shutdown. Repeated limit violations ultimately trigger a complete gear train overhaul. The second strategy introduces a non-ideal preventive maintenance framework (imperfect maintenance), initiated each time the contamination limit is surpassed. This strategy explicitly incorporates the environmental costs attributable to filtration operations. Each such intervention partially reduces the gearbox failure rate, reducing it to an intermediate level situated between the existing deteriorated state and that of a newly commissioned equipment. After N such interventions, full renewal is performed. This approach quantifies and minimizes the environmental costs embedded within each maintenance cycle. Two analytical models are developed. The first determines the economically optimal replacement periodicity and the second jointly optimizes the number of imperfect maintenance action and logistical scheduling. Numerical simulations and sensitivity analysis are conducted to compare both strategies. The results demonstrate that each maintenance strategy can be more advantageous depending on key criteria such as gearbox reliability, preventive maintenance costs and replacement expenses. From a practical standpoint, this work provides wind farm operators with clear, actionable decision rules. The imperfect maintenance strategy significantly reduces total costs compared to conventional periodic replacement. Sensitivity analysis reveals operational thresholds: when filtration costs are high, operators should reduce the number of imperfect interventions; when renewal costs dominate, increasing preventive interventions before full replacement becomes economically beneficial. The study provides wind farm operators with adaptable, optimized solutions tailored to their financial and operational specificities, contributing to more reliable and cost-effective wind energy production.