Understanding Milky Oil in Lower Unit

Last time I’ve discussed water in the lower unit oil but this time I’ll answer the query about milky oil in lower unit. But what to do if there is milky oil in lower unit?

To fix the issue – repair or replace leaking seals or gaskets, replace a worn propshaft seal, and repair or replace any cracked or damaged lower unit housing. Monitor temperature fluctuations, winterize during storage, fix cooling system issues, and replace corroded or damaged components to prevent condensation and coolant leaks.

Want to learn more about it? Keep on reading till the end.

What Does Milky Lower Unit Oil Mean?

Milky lower unit oil refers to the appearance of the oil inside the lower unit of a marine engine. When the lower unit oil appears milky, it indicates the presence of water or coolant contamination. The milky appearance occurs due to the emulsification of oil and water or coolant.

This can be a result of water intrusion through leaking seals or gaskets, a bad propshaft seal, a cracked lower unit housing, condensation from temperature fluctuations or extended periods of inactivity, or a coolant leak from a malfunctioning cooling system or corroded components. Milky oil is a warning sign that there is a problem with the lower unit that requires immediate attention to prevent further damage and ensure proper lubrication.

What Does Milky Lower Unit Oil Look Like

Milky lower unit oil has a distinct appearance that sets it apart from normal, healthy oil. Instead of the usual clear or translucent color, milky oil appears opaque and has a whitish or light gray hue.

It often has a thick, creamy consistency, resembling a milkshake or a mixture of oil and water. In some cases, the milky oil may separate into distinct layers, with water or coolant settling at the bottom and oil floating on top. Regardless of the specific scenario, the presence of a milky appearance in the lower unit oil is a clear indication of water or coolant contamination, highlighting an underlying issue that needs to be addressed.

Milky Oil In Lower Unit: At A Glance

Now let’s learn about the whole thig at a table view. But before that if you want you can cehck an article I covered on – How long can you run an inboard out of water?

Water Intrusion
– Leaking Seals or GasketsRepair or replace seals or gaskets
– Bad Propshaft SealReplace worn or damaged propshaft seal
– Cracked or Damaged Lower Unit HousingRepair or replace damaged lower unit housing
– Temperature FluctuationsMonitor temperature fluctuations and address accordingly
– Extended Periods of InactivityProperly winterize the boat during storage periods
Coolant Leak
– Malfunctioning Cooling SystemFix cooling system issues, repair or replace components
– Corroded or Damaged ComponentsReplace corroded or damaged cooling system components

Causes of Milky Oil in Lower Units

Below I’m adding few causes of milky oil in lower units.

1. Water Intrusion

Leaking Seals or Gaskets: The most common cause of milky oil in the lower unit is water intrusion. Leaking seals or gaskets can allow water to enter the lower unit, mixing with the oil and resulting in the milky appearance.

Bad Propshaft Seal: The propshaft seal, which seals the propeller shaft where it enters the lower unit, can deteriorate over time. A worn or damaged seal can permit water to enter and contaminate the lower unit oil.

Cracked or Damaged Lower Unit Housing: Physical damage to the lower unit housing, such as cracks or holes, can also allow water to enter and mix with the oil, causing it to turn milky.

2. Condensation

Temperature Fluctuations: Extreme temperature fluctuations can lead to condensation inside the lower unit. When warm air enters the unit and then cools down, moisture can accumulate, resulting in milky oil.

Extended Periods of Inactivity: If a boat or marine engine is left unused for an extended period, condensation can occur inside the lower unit due to the lack of warm oil circulating through the system.

3. Coolant Leak

Malfunctioning Cooling System: A malfunctioning cooling system, such as a leaking radiator or a faulty water pump, can cause coolant to mix with the lower unit oil, giving it a milky appearance.

Corroded or Damaged Components: Corrosion or damage to the cooling system components, such as the heat exchanger or hoses, can also result in a coolant leak and contamination of the lower unit oil.

Effects of Milky Oil in Lower Units

Now let’s learn what may happen due to the problem.

1. Reduced Lubrication Efficiency

Milky oil in the lower unit signifies a compromised lubrication system. Water or coolant in the oil reduces its ability to properly lubricate the gears and bearings, leading to increased friction and wear.

2. Increased Wear and Tear on Internal Components

The lack of effective lubrication due to milky oil can accelerate the wear and tear of gears, bearings, and other vital parts in the lower unit.

3. Potential Damage to Gears, Bearings, and Other Critical Parts

Over time, the continuous use of milky oil can cause severe damage to the gears, bearings, and other crucial components within the lower unit, eventually leading to costly repairs or replacements.

Milky Oil in Lower Unit: Diagnosis and Inspection

There are 2 steps to complete this task. Let’s learn about it.

Visual Inspection

Checking for Water or Coolant Presence in Oil: By examining the lower unit oil, one can look for the presence of water or coolant. If the oil appears milky or has separated into distinct layers, it indicates a problem.

Observing Oil Color and Consistency: Fresh lower unit oil should be clear, with a consistent color and viscosity. Any deviation from the normal characteristics may indicate water or coolant contamination.

Performing a Pressure Test

Verifying the Integrity of Seals and Gaskets: A pressure test can help identify potential sources of water or coolant intrusion by assessing the integrity of seals and gaskets.

Identifying Potential Sources of Water or Coolant Intrusion: By pressurizing the lower unit, technicians can detect leaks and pinpoint the areas that require repair or replacement.

What To Do If Lower Unit Oil Is Milky: Remedies And Maintenance

It’s time for you as I’ve come up with the solutions. Don’t skip this part.

1. Drain and Replace the Milky Oil

The first step in addressing milky oil is to drain the contaminated oil completely. Ensure that the oil is disposed of properly, as it may be considered hazardous waste. After draining, replace it with fresh, recommended lubricant.

2. Repair or Replace Damaged Seals, Gaskets, or Components

Identify and fix the sources of water or coolant intrusion. This may involve repairing or replacing damaged seals, gaskets, or components to prevent future oil contamination.

3. Properly Flush and Refill the Lower Unit with Fresh Lubricant

After resolving the underlying issue, flush the lower unit with fresh lubricant to ensure any remaining contaminants are removed. Follow the manufacturer’s instructions for the correct type and amount of lubricant to use.

4. Regularly Inspect and Maintain Cooling System Components

Implement a regular maintenance schedule to inspect and maintain the cooling system. Address any issues promptly to avoid coolant leaks and subsequent milky oil problems.


I bet you might never want to face this problem again. Don’t worry here’s a prevention guide for you.

  • Regular Maintenance and Inspection Schedule: Adhere to a routine maintenance and inspection schedule to identify potential problems early on and prevent milky oil issues.
  • Ensuring Proper Sealing and Gasket Integrity: Regularly inspect and replace seals and gaskets to maintain their integrity and prevent water or coolant intrusion.
  • Monitoring and Addressing Temperature Fluctuations: Be mindful of temperature fluctuations and take necessary measures to prevent condensation inside the lower unit. This includes properly winterizing the boat during storage periods.
  • Following Manufacturer’s Guidelines for Oil and Lubricant Use: Always use the recommended oil and lubricants specified by the manufacturer to ensure optimal performance and prevent potential issues.


That’s all about milky oil in the lower unit. It is a clear indicator of a problem that requires immediate attention. I think you are now clear about the problem. It’s time to say bye bye but again suggesting our another article on – Suzuki 200 HP outboard problems

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