# Outboard Gearbox Oil: Checking for Water and Changing It **By Sawley's Auto and Marine** · 2026-09-10 The gearbox on an outboard runs in a few hundred millilitres of oil, underwater, on the far side of two small seals from the sea. It is the most exposed lubricated component on the boat and the one owners look at least often. When it fails, it fails expensively, and almost every gearbox we rebuild at Kadina had been warning its owner for a season through the colour of that oil. ## What is the gearbox oil telling you? Pull the lower drain screw and catch the first dribble in something white. There are only a few things it can look like. - **Clear amber or brown.** Healthy. Darker with hours, which is normal. - **Milky, creamy, coffee-coloured.** Water in the oil. A seal has let go somewhere and seawater has been emulsifying with the lube every time the gears turn. The bearings are already living on borrowed time. - **Water first, then oil.** Worse than milky. Enough water has come in to settle out as a separate layer at the bottom, which means the leak is not small and the gears have been running through it. - **Metallic sheen, or fuzz on the magnetic drain screw.** A fine grey haze is normal wear. Glitter, or flakes you can feel, is a gear or bearing breaking up. - **Smells burnt.** The oil has been overheated, usually from running low. ### Where the water gets in There are two seals that matter. The **prop shaft seal** sits directly behind the propeller, and its usual enemy is fishing line: a few metres of braid wound tight around the shaft will saw through the seal lip in a single trip. The **shift shaft seal** at the top of the gearcase perishes with age. Either can let water in without dripping a drop of oil out, which is why the oil looks fine on the dipstick you do not have and milky on the drain screw you do. Pull the propeller once a season and look at the shaft behind it. Line wrapped there is a seal on its way out, and cutting it off before it does the damage is a two-minute job. The rest of the propeller inspection is in our guide to [choosing and checking a propeller](/blogs/guides/choosing-a-propeller-pitch-diameter-material). ### Changing it The routine is the same on almost every outboard. With the leg vertical, remove the lower drain screw first and then the upper vent screw, and let the old oil drain completely into a container. Then fill _from the bottom_: screw the pump or squeeze bottle into the lower hole and pump until clean oil runs out the upper vent. Fit the upper screw, then quickly withdraw the pump and fit the lower one. Filling from the bottom is what pushes the air out ahead of the oil; fill from the top and you trap a pocket that leaves the top bearing dry. Two rules. **Replace both sealing washers every time** — they are a few cents and a reused one weeps. And **use a proper marine gear lube**, a hypoid oil made for the job; the 75W-90 synthetic [Penrite Marine Gear Oil](/products/mar7590005-marine-gear-oil-75w-90-500ml) suits most modern legs, and for a Yamaha the [genuine Yamalube gear case lube](/products/90790-bs811-gl5-gear-case-lube-750ml) is the factory fill. Engine oil is not a substitute; it lacks the extreme-pressure additives that gear teeth need. How much it holds varies from a couple of hundred millilitres on a portable to close to a litre on a big V6. We have listed the gearcase capacity for hundreds of outboards, alongside the crankcase figures, on the [oil capacity charts](/pages/outboard-sterndrive-oil-capacity-charts). ### How often Once a year at minimum, or every hundred hours, and always before winter storage rather than after — drain it in autumn and you find the milky oil in May, when there is time to fix the seal, rather than in October when there is not. If you have had line around the prop, or you have touched bottom hard, drain a sample and look before you trust it for the season. ### When the oil comes out milky Fresh oil will not fix it. The seal has to be replaced, and on most outboards that means pulling the gearcase and the propeller shaft carrier, which is a workshop job with the right pullers and a pressure tester. Pressure testing is the important part: a rebuilt gearcase is pressurised and left to sit, and if the gauge holds, the seals hold. Doing the job without the test is guessing. Sterndrives have the same logic and their own oil; the drive oil question, and which grade each Volvo Penta drive takes, is a separate guide. For outboards, gearcase seal replacement and pressure testing is routine work through [outboard servicing and repairs](/pages/marine-outboard-servicing-and-repairs) at Kadina. If the oil came out looking like a milkshake, ring [0412 532 984](tel:+61412532984) before the next trip, not after. **Tags:** Guides, Outboards, Servicing --- > Source: [Sawley's Auto and Marine](https://www.sawleysautoandmarine.com.au/blogs/guides/outboard-gearbox-oil-checking-for-water)