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#3389827 - 06/05/14 02:29 PM mercrusier 140 3.0l oil type.
leroyd92 Offline


Registered: 01/21/14
Posts: 208
Loc: AZ
theres a really trustworthy marine shop here in my town, and he swears by using 40wt in most the little boat motors and claims ( even after 40 years ive never had a problem with any customers )

ive done some research on oils used for the 3.0 l mercs and they are all over the place. theres people saying theyve used 530 for 10 years and others saying 50. looks like i cant go wrong. currently, i have like 3 quarts of 1540 in it and a quart of 2050. it was of what i had on the shelf honestly... ill run it this season, and maybe change it next year or so.

would i be better off using a 1540 for the boat or could i use a synethic 1030 and be safe? this is an off in the future switch but id like everyones opinion.

my chevy 350 in the volvo has 1540 in it.may just use it for the boats.

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#3389847 - 06/05/14 02:51 PM Re: mercrusier 140 3.0l oil type. [Re: leroyd92]
hemitom Offline


Registered: 03/12/07
Posts: 732
Loc: canada
I had a mercruiser as well and i used 15w40 and quicksilver 25w40 never noticed any difference between the two, so stuck with 15w40.
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#3389880 - 06/05/14 03:35 PM Re: mercrusier 140 3.0l oil type. [Re: leroyd92]
bullwinkle Offline


Registered: 10/09/04
Posts: 3890
Loc: Cincinnati, OH, USA
I use Mobil Delvac 1300 15W40 in my Mercruiser 350 Mag, can't imagine Merc's 25W40 would be better.
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#3389929 - 06/05/14 04:10 PM Re: mercrusier 140 3.0l oil type. [Re: leroyd92]
leroyd92 Offline


Registered: 01/21/14
Posts: 208
Loc: AZ
yeah might as well stay with 1540. i put full synthetic in my truck and thought for the cost might put it in something that works hard. but im sure the diesel oil will do just fine

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#3389960 - 06/05/14 04:37 PM Re: mercrusier 140 3.0l oil type. [Re: leroyd92]
Michael_P Offline


Registered: 08/20/07
Posts: 958
Loc: Ohio
When the 3.0 was rated at 140HP, it was almost always speced for 40wt. My old 3.0 135HP said in the owners manual to use 40wt over 50 degrees Fahrenheit. Seeing multi-viscosity oils are more shear stable, you would be fine using 15W40.

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#3390173 - 06/05/14 09:17 PM Re: mercrusier 140 3.0l oil type. [Re: leroyd92]
Donald Offline


Registered: 03/21/04
Posts: 13173
Loc: Upstate NY
I would use a 15W40 or Mercruiser 25W50.
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#3390219 - 06/05/14 10:17 PM Re: mercrusier 140 3.0l oil type. [Re: leroyd92]
Clevy Offline


Registered: 11/11/10
Posts: 6988
Loc: Saskatoon canada
Due to the way a marine engine operates,higher rpm for potentially long periods of time they can elevate oil temps,which is why I feel an hdeo is the best choice.
A 15w-40 is always a good bet. Few VII and decent HT/HS which should keep an oil film on the parts even when the oil is really hot.
A motorcycle oil should do well too. Over priced though
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#3393167 - 06/09/14 04:10 PM Re: mercrusier 140 3.0l oil type. [Re: leroyd92]
philipp10 Offline


Registered: 05/09/14
Posts: 3
Loc: Minnesota
I am greatly confused by the logic of using 15W-40 in a 3.0 Mercruiser. The Mercury manual states 25W-40. Now assuming the Mercury engineers did some testing, it sure seems to me they are pointing poeple to use a heavier weight oil that 15W. Since boats are used when it is fairly warm (I boat in the summer only), the heavier weight makes sense to me.

The only place I might differ from Mercury's recommendation is that they spec out a 25W weight oil. Clearly they are recommending something that is not available except from Mercury of course. If it was up to me, I would use a straight 30 weight.


Edited by philipp10 (06/09/14 04:24 PM)

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#3393172 - 06/09/14 04:21 PM Re: mercrusier 140 3.0l oil type. [Re: philipp10]
Clevy Offline


Registered: 11/11/10
Posts: 6988
Loc: Saskatoon canada
Originally Posted By: philipp10
I am greatly confused by the logic of using 15W-40 in a 3.0 Mercruiser. The Mercury manual states 25W-40. Now assuming the Mercury engineers did some testing, it sure seems to me they are pointing poeple to use a heavier weight oil that 15W. Since boats are used when water is not frozen, the heavier weight makes sense to me.

The only place I might differ from Mercury's recommendation is that they spec out a 25W weight oil. Clearly they are recommending something that is not available except from Mercury of course. If it was up to me, I would use a straight 30 weight.




A. 25w-40 will have little to no VII which means more shear resistance which is likely desirable considering a marine engine tends to rev higher for longer periods of time.
I'm sure a straight grade would do well too when ambient temps allow their use
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#3393422 - 06/09/14 08:33 PM Re: mercrusier 140 3.0l oil type. [Re: philipp10]
OVERKILL Online   content


Registered: 04/28/08
Posts: 25640
Loc: Ontario, Canada
Originally Posted By: philipp10
I am greatly confused by the logic of using 15W-40 in a 3.0 Mercruiser. The Mercury manual states 25W-40. Now assuming the Mercury engineers did some testing, it sure seems to me they are pointing poeple to use a heavier weight oil that 15W. Since boats are used when it is fairly warm (I boat in the summer only), the heavier weight makes sense to me.

The only place I might differ from Mercury's recommendation is that they spec out a 25W weight oil. Clearly they are recommending something that is not available except from Mercury of course. If it was up to me, I would use a straight 30 weight.


They are spec'ing a 40-weight oil not a 25 or 15. The number in front of the W is the winter rating for the oil.

For a conventional oil, as Clevy noted, the 15w-40 probably has more VII's in it than a 25w-40 which could be made without them. That said there are probably a few synthetic 15w-40's and 10w-40's that are made without VII's either.

The 15w-40 BTW may actually be heavier than the 25w-40 depending on what end of the visc scale the Merc oil is on. A 5w-40 HDEO for example is heavier than a Euro-spec 5w-40.
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#3393475 - 06/09/14 09:18 PM Re: mercrusier 140 3.0l oil type. [Re: OVERKILL]
philipp10 Offline


Registered: 05/09/14
Posts: 3
Loc: Minnesota
I'm definitely no expert in oil. So your saying a 15W is more viscous than a straight 30 or 40? And what is "VII" ?

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#3393514 - 06/09/14 09:52 PM Re: mercrusier 140 3.0l oil type. [Re: philipp10]
OVERKILL Online   content


Registered: 04/28/08
Posts: 25640
Loc: Ontario, Canada
Originally Posted By: philipp10
I'm definitely no expert in oil. So your saying a 15W is more viscous than a straight 30 or 40? And what is "VII" ?


No, don't look at it that way, the 15W is just the winter rating of the oil. You can have 15w-40, 15w-50...etc. It just means the lubricant passes the cold performance requirements to carry the 15W designation.

The number AFTER the W is the number we are concerned with here, which is a 40. Now of course there are a range of viscosities that fall under the 40 umbrella, so you can have an oil that is on the thin side of a 40 or the heavy side of a 40, both still 40's, hence my European example from my earlier post.

An SAE 30 is an SAE 30, meaning its viscosity at 100C falls within the range for a 30 as defined by the SAE, same process applies with an SAE 40. A 15w-40 falls inside the same 40 range, but since it is an HDEO, it may have a higher HTHS (it is actually a bit heavier) than your SAE 40. Of course what it offers is improved cold performance over the straight-weight.

Now, VII's, they are Viscosity Index Improvers. Polymers that expand and contract that effect the operational viscosity of the fluid and improve cold flow performance (cause the oil to thicken less) and increase operational viscosity (cause the oil to thin less as it gets hot). These are used in multi-grade oils to make them meet both the cold temp viscosity target and the 100C visc target.

Where things start to get a bit sideways is when you add synthetic oils and modern base stocks to the mix. Many modern base oils (and base oil blends) have a naturally high VI (Viscosity Index) that allows them to meet both the 100C requirement AND the cold temp requirement without the addition of polymers. AMSOIL makes a 10w-30 that falls in that category, Redline's 5w-30 is another example. These products have NO VII's in them (like your SAE 30 or SAE 40) but are able to satisfy the 10W and 5W cold temp performance requirements respectively, hence being labelled as they are.

Other oils can get away with very little VII being added to the mix to meet a relatively wide visc spread (M1 0w-40 for example).

Now, VII's can (and do) shear. What this does is reduce the operational viscosity. That said, a 15w-40, by being an HDEO (Heavy Duty Engine Oil) and spec'd for high stress diesel engine applications, many of which spend a great deal of their time at wide open throttle, is not all that likely to shear. These oils tend to be very shear-stable.

The idea by an OEM to spec a narrow visc range is usually to deal with operational viscosity loss, often due to shear, fuel dilution...etc. A "normal" 10w-40 may not hold its viscosity well enough and perhaps ended up down in the bottom of the 30-grade territory in Merc's testing. This may have been why they spec'd the visc they did, which is likely guaranteed to have no VII's, so it is less likely to shear.

IMHO, a 15w-40 is still a safe bet and probably won't shear either. It may actually be a heavier oil (as I noted earlier) than the spec lubricant anyway.
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#3393755 - 06/10/14 08:15 AM Re: mercrusier 140 3.0l oil type. [Re: Clevy]
kschachn Offline


Registered: 12/26/05
Posts: 2320
Loc: Upper Midwest
But not a straight-weight 30 though, it would be a straight 40.

Why you would want to intentionally do that I'm not sure, given the availability of modern multi-weights.

Originally Posted By: Clevy
Originally Posted By: philipp10
I am greatly confused by the logic of using 15W-40 in a 3.0 Mercruiser. The Mercury manual states 25W-40. Now assuming the Mercury engineers did some testing, it sure seems to me they are pointing poeple to use a heavier weight oil that 15W. Since boats are used when water is not frozen, the heavier weight makes sense to me.

The only place I might differ from Mercury's recommendation is that they spec out a 25W weight oil. Clearly they are recommending something that is not available except from Mercury of course. If it was up to me, I would use a straight 30 weight.

25w-40 will have little to no VII which means more shear resistance which is likely desirable considering a marine engine tends to rev higher for longer periods of time.
I'm sure a straight grade would do well too when ambient temps allow their use
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#3397064 - 06/13/14 11:00 PM Re: mercrusier 140 3.0l oil type. [Re: leroyd92]
friendly_jacek Online   content


Registered: 05/04/03
Posts: 5091
Loc: southeast US
I used 25W40, 40, and 15W40 in my 3.0 Mercruiser. They all had similar oil pressures. But 15W40 was way easier to drain during pre-winter winterization. I use 15W40 now.

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#3400933 - 06/18/14 08:28 PM Re: mercrusier 140 3.0l oil type. [Re: kschachn]
philipp10 Offline


Registered: 05/09/14
Posts: 3
Loc: Minnesota
I had chosen straight 30 thinking it was between the 25 and the 40. It is also Mercury's second choice after the 25W-40. Personally I was figuring the 40 being too thick for cold starts.

What I am still struggling to understand though, is why is Mercury recommending 25W-40 and not 15W-40. What would be the difference? Obviously the 15W-40 is thinner, right? So why then run one over the other?

Secondly, Mercury states that if the 25W-40 is not available, that a good quality straight 30 is recommended. So again, why are so many posters recommending the 15W-40? Wouldn't Mercury's engineers know better then some guy that posts on the internet?


Edited by philipp10 (06/18/14 08:32 PM)

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