Why is my outboard gear oil milky?

Milky gear oil / water in lower unit · Symptom: Creamy/white gear lube when drained.

Quick answer: Most often: prop shaft seal damaged by fishing line, worn seals, drain/vent screw gasket bad. Start here: Check prop shaft for fishing line. If that doesn't fix it, work through the checks below, and see a marine mechanic if you're unsure.

Likely causes (most to least common)

1. Prop shaft seal damaged by fishing line
2. Worn seals
3. Drain/vent screw gasket bad

Safe owner checks & fixes

1. Check prop shaft for fishing line.
2. Have dealer pressure-test gearcase.
3. Replace gear lube.

Parts / tools

Gear lube pump, gaskets, prop wrench.

Call a marine mechanic when

Always: gearcase pressure/vacuum test and seal replacement; metal flakes in lube. If you're on the water and unsafe, call TowBoatUS/Sea Tow or the Coast Guard (VHF 16).

Sources

• https://www.mercurymarine.com/us/en/lifestyle/dockline/how-to-winterize-your-outboard
• https://dwr.virginia.gov/blog/15-tips-for-winterizing-your-boat-and-motor/

Values not stated here are model-specific; check your owner's manual.

Related problems

• Why is my outboard hard to shift or not shifting?
• How do I winterize an outboard motor?
• What do I do if my outboard was submerged?