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SAE 90 vs 80W 90 Marine Gear Lube SAE 90 and 80W 90 marine gear lubes can have similar viscosity at operating temperature, but they are not always interchangeable. The correct choice depends on the lower unit design, manufacturer guidance, and whether the oil is specifically formulated for marine use. Lower units operate in harsh conditions. They are submerged, exposed to water intrusion risk, and experience high load at low speeds. Using the wrong gear lube can lead to accelerated wear, corrosi …