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Synthetic oil cause oil dilution?

Old 09-21-2020, 07:56 AM
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Default Synthetic oil cause oil dilution?

I just bought my first 4-stroke,new. Little 20hp Tohatsu all the bells and whistles so the wife can use in on the 14' utility boat. Like a good doobie, I got on a few forums to check it out and I read a post about not using synthetic oil in this engine because it causes oil dilution (making oil). The post was by a Tohatsu tech. Sounds a bit far-fetched, but the guys works on these engines for a living and should know. Maybe it's because synthetic oil comes with the reputation of extending intervals and there are folks that don't change when they should on an outboard. The thing tops out at 6100 RPM and there are guys that run them WOT all the time. I would think a synthetic is the way to go after break-in. Whats the board think?
Old 09-21-2020, 08:04 AM
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Originally Posted by tucker99 View Post
I just bought my first 4-stroke,new. Little 20hp Tohatsu all the bells and whistles so the wife can use in on the 14' utility boat. Like a good doobie, I got on a few forums to check it out and I read a post about not using synthetic oil in this engine because it causes oil dilution (making oil). The post was by a Tohatsu tech. Sounds a bit far-fetched, but the guys works on these engines for a living and should know. Maybe it's because synthetic oil comes with the reputation of extending intervals and there are folks that don't change when they should on an outboard. The thing tops out at 6100 RPM and there are guys that run them WOT all the time. I would think a synthetic is the way to go after break-in. Whats the board think?
My 300 Yamahas made oil right up to the day I switched to dino! I broke them in by the book! Won't do that again either! Now I'm at about 26x hrs and oil level stays on the mark! No more synthetic for me...
Old 09-21-2020, 08:06 AM
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Thanks, wondering how that can happen. What oil are you using now?
Old 09-21-2020, 08:09 AM
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In a car/truck engine with a 210F thermostat and much higher oil temps than any raw water cooled marine engine synthetic makes some sense but for an outboard the cheapest SN rated dino oil of appropriate viscosity is beyond good enough. And don't buy into the FC-W hoopla.
Old 09-21-2020, 08:13 AM
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Originally Posted by tucker99 View Post
Thanks, wondering how that can happen. What oil are you using now?
I'm in warm water and use Castrol 20/50
Old 09-21-2020, 10:53 AM
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Originally Posted by HTJ View Post
In a car/truck engine with a 210F thermostat and much higher oil temps than any raw water cooled marine engine synthetic makes some sense but for an outboard the cheapest SN rated dino oil of appropriate viscosity is beyond good enough. And don't buy into the FC-W hoopla.
Makes sense, but that only explains why it isn't necessary to use synthetic, not why synthetic is bad for a 4-stroke outboard motor.
Old 09-21-2020, 11:00 AM
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Synthetic lubricant is NOT bad for any engine, in fact it's superior. However, breaking in the engine with a true 100% synthetic is a bad idea. The rings will not seat or bed in properly due to a lower frictional coefficient. Break in the engine well with cheap lube, when the engine stops "making oil", you can switch to a synthetic. Just remember, not all synthetics are created equal, but that's another topic entirely.
Old 09-21-2020, 12:01 PM
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The oil choices most associate as synthetic here in the US are not much different than conventional. They all start from crude oil and synthetic just refers to the fact there are processed and refined more. True synthetics that use PAO ester bases would not be good in the marine environment, but most synthetic oils in the US are not that.
So if using Mobil 1 as an example, it would not cause the engine to make oil any more than a conventional equivalent viscosity oil.
Old 09-21-2020, 12:24 PM
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What is the prevaling theory as to how synthetic oils “make” oil? This is a new one to me.
Old 09-21-2020, 12:35 PM
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Originally Posted by Fishinado View Post
What is the prevaling theory as to how synthetic oils “make” oil? This is a new one to me.
In simple terms the synthetics are too good at their job and don’t allow enough frictional heat to properly seat the rings. Fuel blows by the ring and dilutes the oil. In theory I guess if you could control the thermostat to let the engine run hotter, that could help to seat the rings but no ones going to do that.
Old 09-21-2020, 12:37 PM
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Originally Posted by Fishinado View Post
What is the prevaling theory as to how synthetic oils “make” oil? This is a new one to me.
Just type "Yamaha making oil" in the search bar have some cold beverages ready and read for days! The people that say it doesn't happen just never had it happen to them!
Old 09-21-2020, 12:44 PM
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Originally Posted by HTJ View Post
In a car/truck engine with a 210F thermostat and much higher oil temps than any raw water cooled marine engine synthetic makes some sense but for an outboard the cheapest SN rated dino oil of appropriate viscosity is beyond good enough. And don't buy into the FC-W hoopla.
Suzuki still calls for SL ... go figure. and some API's that would be considered obsolete. But agree anything over 10w+ and syn oil is not making any improvements. Start getting into thinner oils, motors with tight tolerances and it becomes important.


Originally Posted by Fishinado View Post
What is the prevaling theory as to how synthetic oils “make” oil? This is a new one to me.

Does not matter if it is Dino, Synthetic or blended. Correct API rating and weight is what is important. How the oil is made is not important, and just FYI no two synthetic oils are made the same.

Originally Posted by Vantaredoc View Post
Just type "Yamaha making oil" in the search bar have some cold beverages ready and read for days! The people that say it doesn't happen just never had it happen to them!

Is Yamaha 10w-30 motor oil Synthetic? can not recall

Last edited by Jeepman; 09-21-2020 at 12:56 PM.
Old 09-21-2020, 01:03 PM
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Originally Posted by Jeepman View Post



Is Yamaha 10w-30 motor oil Synthetic? can not recall
I don't know what it is. I will never run anything that says Yamalube again.
Old 09-21-2020, 01:10 PM
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Holy Aleemada, another oil thread. Gotta get the popcorn ready for the long haul.
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Old 09-21-2020, 01:18 PM
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Not an outboard BUT i had a 2001 GMC truck bought new. Used Mobil1 since new and always had a little piston slap and valvetrain float/rattle. GM assured me it was not a problem and gave me a 100K mile engine warranty.
I figured why waste money on M1 when I had a 100K warranty. Switched to premium dino oil and Presto, ALL ticks and rattle went away.
Old 09-21-2020, 01:31 PM
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All I asked was why/how does synthetic oil cause dilution. Don't care about brands or starting an oil thread.

1-I've heard that the rings do not seat properly. So that means we go back to the old ways and break in with conventional and run synthetic for the long haul?

2-Outboard thermostats can't regulate a temp hot enough to make synthetics work and they somehow have an adverse effect?

3-Mostly happens with Yamaha's but they can all get it.

Good grief, I've been glancing over making-oil threads for years and we've never figured out how or why it happens? Hope I'm not going to regret buying this little 4-stroke.
Old 09-21-2020, 02:13 PM
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Originally Posted by tucker99 View Post
All I asked was why/how does synthetic oil cause dilution.
It doesn't.
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Old 09-21-2020, 02:30 PM
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Many of the threads involving making oil pertain to Yamaha although I believe the issue is not limited to Yamaha. I believe the issue revolves around a super hard plasma coating that is being used to replace the cast iron cylinder liners used in the past. https://global.yamaha-motor.com/busi...gy/processing/My understanding is that synthetic oil is too slippery to allow the rings to properly seat to this super hard surface. What I have personally done is used conventional oil for the first 300 hours making certain that my motors were broken in properly and not making oil. After 300 hours I switched to Amsoil full synthetic and have not seen any indication of making oil. So in your post above I would choose option #1.
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Old 09-21-2020, 02:33 PM
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Be hard pressed to blame Yamahas making oil problems on synthetic oil due the fact most Yamahas do not use synthetic oil....
Old 09-21-2020, 03:32 PM
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Synthetic oil is hydroscopic. It has an affinity for and will tend to attract combustible hydrocarbons (gasoline) to itself.

Viscous hydrocarbons (lubricating oils) do not attract combustible hydrocarbons.
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