4t outboard W rating question

[1978elcamino]

Marine 4t oil typically has a W rating of 20 or 25. Just wondering why compared to automotive oils. Seems odd because most boats are slid of the trailer, started and ran for a few minutes before running hard the rest of the day. One would think a lower W rating would protect better upon start up being thinner and get lube circulating quicker. Many boaters use rotella t 15w40 or a simular diesel multigrade oil, some using t6 5w40 also. I just cant imagine cranking up a $20K outboard in Alaska with 25w40 and blasting off to 4500rpm when the oil is what ever the ambient temperature is. I bet the oil viscosity decals are the same in Florida and Alaska. Can someone shed a bit of light on this for me?

 

[dwendt44]

Another point: Depending on age, outboard motors have a 140° thermostat.
Not even sure that a 40 weight oil would be optimal.


My 2¢

 

[Snagglefoot]

This. I've tried to save this PDF of a chart from Kew Engineering showing viscosity (cSt) vs temp ( deg C).

While they don't have 20w40 graphed they have 20w50 and 15w40. In the range of 15 C to 30 C, where most boating probably takes place (correct me if I am wrong) the vis of 5W and a 10 W are very close at about 250 cSt. A 15W would be closer to 400 cst and a 20 W would be at 500 cSt. You are correct that the lower W rating will give you a lower vis during the start-up.

But compare this to starting a car motor at -30 degrees, where a 5W would be at 6,000 sCt and still fires up fine.

I think the 20w40 is spec'd to give you the maximum performance and resistance to shearing at the high rpm operating temperature. The higher vis during a "cold" start is not significant enough to worry about in the manufacturer's opinion.

Hope this helps.

 

[CT8]

How would a lower viscosity protect better when starting unless the oil was too cold to pump?

 

[krismoriah72]

Originally Posted by 1978elcamino
Marine 4t oil typically has a W rating of 20 or 25. Just wondering why compared to automotive oils. Seems odd because most boats are slid of the trailer, started and ran for a few minutes before running hard the rest of the day. One would think a lower W rating would protect better upon start up being thinner and get lube circulating quicker. Many boaters use rotella t 15w40 or a simular diesel multigrade oil, some using t6 5w40 also. I just cant imagine cranking up a $20K outboard in Alaska with 25w40 and blasting off to 4500rpm when the oil is what ever the ambient temperature is. I bet the oil viscosity decals are the same in Florida and Alaska. Can someone shed a bit of light on this for me?


How it was explained to me.. Marine 25w40 has no viscosity improvers which resists shearing. It is FC-W certified so it has a lot of corrosion and anti-wear protection.

I think the engines experience higher rpms and loads than normal car engines.. would probably be like towing heavy all the time in a normal engine.

With that being said I think 15w50 would do fine, not sure about the corrosion protection.

 

[A_Harman]

Originally Posted by CT8
How would a lower viscosity protect better when starting unless the oil was too cold to pump?


Shorter delay of getting oil pressure to the farthest reaches of the oil system.

 

[CT8]

Originally Posted by A_Harman
Originally Posted by CT8
How would a lower viscosity protect better when starting unless the oil was too cold to pump?


Shorter delay of getting oil pressure to the farthest reaches of the oil system.

Positive displacement oil pump. If the oil pumps it will get to the farthest reaches of the engine.

 

[Snagglefoot]

Originally Posted by CT8
Originally Posted by A_Harman
Originally Posted by CT8
How would a lower viscosity protect better when starting unless the oil was too cold to pump?


Shorter delay of getting oil pressure to the farthest reaches of the oil system.

Positive displacement oil pump. If the oil pumps it will get to the farthest reaches of the engine.


Two things though. The positive displacement pump is connected to a tube into the sump that is fed by atmospheric pressure. Secondly, simple you tube videos show the increase in time it takes for oil to reach the rockers. Colder higher vis oil takes longer. The efficiency of the pump is affected with less oil entering the pump per rotation.

 

[Garak]

Under ordinary temperatures, the oil will flow just as quickly if it's a monograde as if it were a 0w-XX. The oil pumps in the ambient temperature, or it does not.

 

[A_Harman]

Originally Posted by CT8
Originally Posted by A_Harman
Originally Posted by CT8
How would a lower viscosity protect better when starting unless the oil was too cold to pump?


Shorter delay of getting oil pressure to the farthest reaches of the oil system.

Positive displacement oil pump. If the oil pumps it will get to the farthest reaches of the engine.


Positive displacement pump with pressure relief valve. High viscosity causes high oil pressure, relief valve blows, dumps oil before it can get to the farthest reaches of the engine. The big engines we worked on at Cummins had a two minute allowance for getting oil to the overhead. If not, shut the engine down. Also, I count the seconds that it takes to get oil pressure in my B5.9, and if I have let the truck sit for more than 2 days, it takes a few seconds to get oil pressure if ambient temperature is 60-ish F. Below freezing, and if I have let the truck sit, I can count to 20 before oil pressure indicates. Why? The oil pump has a long supply tube, and it drains out over a couple of days. It takes some time for the pump to reestablish prime.

 

[philipp10]

The idea that a cold start happens when water is not ice makes me laugh. Dude, anything above 32 f is warm in my book. I live in Minnesota....light weight oils like 0w, 5w or even 10w are for well below freezing. The reason boats like heavier weights is because they are run at full torque pretty much all the time. Heavy oils help protect the bearings and wear surfaces better than this watery crap that is really only applicable for below about 20 degrees F.

 

[4WD]

NMMA also tests for anti foaming due to long runs at WOT.

That no VM 25-thick is a big deal to Mercury.
However … one of the big sellers in 4 stroke is Yamaha … Yamalube conventionals are 10w30 … and the Synthetic recommended for the big SHO engines is 5w30.
Different strokes for different boats …