Sierra 25W-40 4-Stroke Synthetic Marine Oil Sierra 4-Stroke 25W-40 Engine Oil is blended with premium grade synthetic and mineral based stock combined with superior designed additives formulated for the stress endured by sterndrive and inboard engines. This oil was developed for the marine environment and will outperform standard 4-cycle automotive engine oils. Sierras 25W-40 oil offers excellent detergency, anti-oxidants, anti-wear, and corrosion protection. This oil provides numerous benefits