Re: Lower Unit Gear Lube API GL-5 Will Any work?
API GL-5 is an extreme pressure lubricant. It is typically use in gearcases where a lot of horsepower is transmitted. Automotive rear ends are an example. The manufacturers add sulfur compounds (among other things) to base stock oil to acheive this rating. That's why rear end oil or lower unit oil has that distinctive smell. Smell it once and you will never forget it.
I have friends who insist that there is no difference in oils and automotive rear end oil is perfectly suitable for lower units. Automotive rear ends are not expected to get water in them, and indeed, this oil is OK if you are sure you will never get water in the lower unit. However, the only way to ensure this is to keep the boat on the trailer.
Marine lower unit oil has more corrosion inhibitors and so is more suited to the use in water. Since marine lower units can and do get water in them, it is also best to change oil once a year. End of season is best so you don't overwinter with water in the gearcase.
