4 stroke oil looks milky

[chinook35]

🤞 it’s not freeze damage   

[greenhornet73]

I really hope not

[fisherdude]

I got tohatsu when I had carb problems it filled the crank case with gas. Like you said pulled dip stick milky oil poor out carb rebuild oil change x 2 was all set.  I don't under stand how a stuck open thermostat would flood the crankcase if it's water in the crankcase like others have said you might have issues with the block or head good luck. 

[Pair of Jacks]

Another point to be aware of  for Mercury 4-stroke outboards is Mercury specifically recommends lower viscosity oil during cold weather operation.  I learned the hard way with my 2019 Mercury 25 EFT 4-stroke. I fish year around here on my home port on Lake Anna Virginia and last winter (January 2023) we headed out on a 35 degree day and before we ever left the slip the engine alarmed and shut down.  Fortunately the engine shut down before any real damage…took the boat to my dealer…and he fired up the motor inside his maintenance bay (68 degrees) and no issues.  Then he checked the oil change history and eureka the oil was the higher viscosity warm weather  oil.  So I learned a valuable lesson…read the owners manual wrt different operating conditions and different oil viscosity.  

[13owhunter]

Mentioning a warm or cold weather oil seems strange to me.  Almost every modern engine uses multiweight oil which I would think just about any common flavor should do fine in 35 degree weather.  Wonder if your oil pressure sensor is just a bit sensitive....  

[Pair of Jacks]

[smfishparadise]

Sounds like a head gasket to me.

[greenhornet73]

I replaced the thermostat and changed the oil twice.  Will fill again tonight and run it and post what I find.

[greenhornet73]

I ran a third quart of oil through the crankcase today to make sure all of the gas/oil mixture was out and filled it with good oil. Fired it up and ran it for 10 minutes on almost half throttle with muffs on.  Checked the oil after and was perfectly clean and level didn’t change.  I’ll run it on the lake this weekend but for now it seems replacing the thermostat did the trick.  Thanks everyone for all the input. If anything changes over the weekend I will post it to help anyone in the future with the same problem.

[greenhornet73]

Just got home from a 3 hour troll and the oil level is still perfect.  Easy fix with a replaced thermostat.

[Pair of Jacks]

Glad is was something inexpensive and simple.

[greenhornet73]

Same here! Thanks for all the responses and troubleshooting help.