I want to “top off” my lower unit gear housing with Sierra Synthetic Blend Hi-Performance lubricant. Will it be compatible with the engine manufacturer lube currently in the housing?
It depends. Sierra blends all of their gear lubes to meet or exceed the engine manufacturer specification. However, you should never mix organic, semi-synthetic or full synthetic gear lubes. If you are 100% certain that the gear lube in the lower unit is also a semi-synthetic, then yes you can just “top off”. However, it is always best to drain and then replace the old gear lubricant with fresh lube. In most cases changing lower unit lube is easily done by the boat owner and Sierra also offers an assortment of gear lube pumps and adaptors to make the job easy and hassle free. Remember to replace the gaskets or o-rings on your drain and vent screws and inspect the old lubricant for metal chips and water (usually a milky or streaked appearance to the lube) that may indicate a problem in the lower unit. While you are down there, pull your prop and inspect the prop shaft for fishing line that can cut a seal and apply Sierra spline grease (18-9200) to the propshaft to prevent corrosion.
