mercruiser 4.5l 250hp with catalytic converter

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Apr 16, 2010
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34
Location
tx usa
My new to me boat is a 2017 chapperal with the mercruiser 4.5l 250hp. This motor had the option to come with catalytic converter or without. Mine HAS the cat .....Now from my research the oil it takes is the mercury 25W-40. And my research also said that marine oil has additives that are no good for cats, so I'm now I'm not sure what oil to use. I wanted to go with M1 extended performance/ high mileage Extended performance like I use in all my equipment but now I see they don't make a 40 weight version. So I was going to go with just M1 10W-40....

Previous owner always had the marina do all the maintenance with OEM recommended oils,lubes,seals etc. 300 hours on the boat. Oil changes occured every year so 6 oil changes have already been done

Any insight would be great.
 
Current Mercury oil is fine for cats...which is why I now choose to use VR1 20w-50 on my antique inboard... You will be fine.
If that's the case , then the oil would be the same as the automotive formula at 2 times the price.... Why not just use a 10w-40 or 15w40 auto/diesel oil?

Also that doesn't seem to be 100% accurate since project farm did a test a year ago with both oils finding out it still had additives not good for cats
 
If that's the case , then the oil would be the same as the automotive formula at 2 times the price.... Why not just use a 10w-40 or 15w40 auto/diesel oil?

Also that doesn't seem to be 100% accurate since project farm did a test a year ago with both oils finding out it still had additives not good for cats
There is a lot of info out there on additive levels, but here are a few points to research;
1) additive levels, the potentially cat affecting additives are necessary, the issue is how much. Last UoA I saw for the current Merc/Quicksilver oil had Zn/Ph levels lower than I was comfortable with for an antique.
2) Merc, the OEM, will not sell an oil that 'kills' the catalysts on their current powertrains and their current oil states that it is catalyst compatible
3) The NMMA cert includes corrosion protection and, supposedly, marine oils are optimized for sustained high speed operation. Will high quality auto or m/c oils also meet these specs at a lower price?? Who knows, but you do know that certified marine oils do. I will say that I routinely run our ocean boat (twin 350hp Yamaha O/B's) at 4K+ rpm pushing a 14k lb boat, and I always use Yamaha oil.
4) the relevant info. on all of this is out there from the manufacturers, etc. No real need to listen to the Project Farm, Kilmers, etc. of the world...at least that is my thought.

And lastly, is anyone emissions testing your boat?:D
 
There is a lot of info out there on additive levels, but here are a few points to research;
1) additive levels, the potentially cat affecting additives are necessary, the issue is how much. Last UoA I saw for the current Merc/Quicksilver oil had Zn/Ph levels lower than I was comfortable with for an antique.
2) Merc, the OEM, will not sell an oil that 'kills' the catalysts on their current powertrains and their current oil states that it is catalyst compatible
3) The NMMA cert includes corrosion protection and, supposedly, marine oils are optimized for sustained high speed operation. Will high quality auto or m/c oils also meet these specs at a lower price?? Who knows, but you do know that certified marine oils do. I will say that I routinely run our ocean boat (twin 350hp Yamaha O/B's) at 4K+ rpm pushing a 14k lb boat, and I always use Yamaha oil.
4) the relevant info. on all of this is out there from the manufacturers, etc. No real need to listen to the Project Farm, Kilmers, etc. of the world...at least that is my thought.

And lastly, is anyone emissions testing your boat?:D
Good info thanks. And no , no one is emission testing the boat but if the oil is killing the cats , by killing I'm assuming clogging them, I don't want that to happen .
 
Mercruiser (and Volvo Penta) are both reputable companies and would not sell oil that would damage the catalytic converters in their engines.

One issue you will find with engines catalytic converters is that the exhaust manifolds and risers are only available from the manufacturer. None from Sierra or Barr, etc.

I have a catalytic converters in my Volvo Penta engine & outdrive. Glad I have closed cooling so the only thing I need to worry about is the riser.
 
If that's the case , then the oil would be the same as the automotive formula at 2 times the price.... Why not just use a 10w-40 or 15w40 auto/diesel oil?

Also that doesn't seem to be 100% accurate since project farm did a test a year ago with both oils finding out it still had additives not good for cats
1. There are more anti corrosion additives in the Merc oil, I’m sure nice to know that metal inside your engine will be better protected especially in a salt water environment.

2. The Merc oil is a straight weight 40 oil not a multi viscosity like the other oils you mentioned. No viscosity improvers/ modifiers to shear. The oil in a marine engine is much more prone to shear using automotive oils.
Marine engines under much more stress than imaginable compared to a car with wheels.
The 25w rating is the oil pump ability at low temperatures.

3. You doing your own analysis of oils based on project farm data compared to the engineering of one of the most successful marine engine makers on planet earth.

I’d much more trust Mercury Marine and just highlighted in #2 why what might seem like rational thinking is often incorrect in the difference between the oils you mentioned compared to a straight weight Mercury marine labeled as 25w40.

I guess the bottom line is what is rated for automotive engines and one is rated for marine engines.


“The first engine manufacturer to develop oils that meet the National Marine Manufacturers Association® (NMMA) FC-W® standards for all four-stroke engines,”


By the way, congratulations on the Chaparral!
They are excellent boats and set a high standard for other boat manufacturers to meet.
Excellent choice.
 
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The risk of poisoning Cat's on an IO is way overblown. Most of us had older cars which ran well in excess of 200,000 miles with the original Cats with (gasp) lots of SL higher zinc oils. Those CATs lived more than 5000 hours behind an engine with high zinc oil. How many hours do you suppose you'll be putting on a boat?
 
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