Why does my boat engine specify 25w-40

[OVERKILL]

Mercury and Volvo buy Chevy blocks and build their own engines with them. It is not a GM engine, only a GM block. So what works in a GM 4.3 engine has nothing to do with what might be appropriate in a Mercury 4.3 engine. I do not run mine hard like many do so I have sometimes use 15w-50 full synthetic. Whether that is a dumb idea or not only time will tell. The engine is only 14 years old. I change the oil once a year since it does not get a lot of hours on it.

I believe some of them are based on marinized crate motors as well, or developed in part with the OEM in question. I say this because some of these "packages" share names with the OEM. Most of them, it's a lot more than just the block that's OE too, for example the Mercruiser 888 was a Ford 302 lopo (non-HO firing order) with car heads, Ford distributor, Ford 2-barrel intake and a Holley 2BBL carb. I assume the cam was a custom marine grind, but most of that engine was just standard Ford stuff.

The PCM GT40 in our '95 Supra appears to be some sort of marine Ford crate motor, has GT40 heads, GT40 intake, EEC-IV ECM that's always in open loop and uses all OE Ford sensors for everything. About the only thing marine specific are probably the camshaft and the exhaust manifolds.

 

[Pequod]

I have used Mobil 1 15w-50 oil in my boat after breaking the engine on 25W-40 Mercruiser oil.
The engine is a Mercruiser 5.7l Horizon engine that is freshwater cooled and I have owned it since new.
Satisfied to say the least with the engine and the oil.

 

[wpod]

Isn’t monograde horrible for engines at cold start? Wouldn’t any engine be better off with a synthetic 10w-40? Or even 5w-40? Not a 25W.

Not when it is warm out

 

[Cousineddy]

Okay then. We should all run 25W in our cars when its warm out. Not the 0W or the 5W. Thanks wpod! I knew that the automakers just want our cars to wear out fast so we buy new ones. I just knew it! Not my car! So grateful Quick Silver followed the science and developed this oil grade and gave the finger to YUGE automotive giants. We need more of this kinda thing these days. Tell you what. Soon as it warms up, I am switching to a 25W in my cars. Probably get to 300k miles instead of just 200k.

 

[alarmguy]

Simple = 25w/40 is a 40 weight oil. Wont "shear" like an automotive oil. The 25w is not "weight" in marine oil or automotive oil. A common misconception even in BITOG. The W rating is "winter" pump-abilty, not weight.

Marine engines are under 100s of times more stress than a automotive engine (even aircraft). No comparison what so ever. A car has wheels to roll. A boat is a barge with no wheels being pushed through water, the stress is enormous. Throw in high RPMs and no mutli speed transmission.

Also coolant temperature is mistakenly for some reason equated to oil temperature. Oil temperature runs much higher and why oil has its own temperature.

(ugh, I see I posted in here like 6+ months ago)

 

[Graham Piccinini]

It's kinda fun watching trolls get the attention they crave.

 

[locomaster1969]

In my experience a 15w40 oil is up to the task and will do the job. M1 15w50 and 10w40 are also good candidates. My first ski boat was a Bayliner Capri 19' with a Volvo Penta I/O. The engine was a Volvo red block 4 cyl. Had a heat exchanger and it worked well. Rode that boat many, many hours. Tubing, cruising, full throttle, donuts, used to make the kids go flying out of the tube and into the sky. Outran a few sudden storms in it too. Best boat I ever had. Great memories. Don't put too much thought into the oil weight. Buy one of the oils I suggested above and enjoy the boat. Be sure to check the oil level each time before you head out on the lake!!

 

[Astro14]

Okay then. We should all run 25W in our cars when its warm out. Not the 0W or the 5W. Thanks wpod! I knew that the automakers just want our cars to wear out fast so we buy new ones. I just knew it! Not my car! So grateful Quick Silver followed the science and developed this oil grade and gave the finger to YUGE automotive giants. We need more of this kinda thing these days. Tell you what. Soon as it warms up, I am switching to a 25W in my cars. Probably get to 300k miles instead of just 200k.

@Cousineddy - What's your intent with this post? I'm asking as a moderator.

It's so out of bounds for either sarcasm, or ignorance, that it deserves to be deleted.

From the perspective of many, it's a troll post.

And we don't allow trolls.

So, please, help me out here - what did you intend?

 

[Cousineddy]

Why would you use a 25w and not a 5w or 10W in a boat engine? The W is the cold viscosity, the 40 is the hot viscosity. Everyone says its because boats work harder, and oil temp runs hotter. Everyone knows that. But when you start and idle a boat cold, its just like any other car engine. So, why isn’t boat oil 5w-40, or 10w-40. Why is it 25w-40? Want a technical reason. Not cause its warm out. Its a simple question. You fancy oil guys don’t know the answer.

 

[kschachn]

Why would you use a 25w and not a 5w or 10W in a boat engine? The W is the cold viscosity, the 40 is the hot viscosity. Everyone says its because boats work harder, and oil temp runs hotter. Everyone knows that. But when you start and idle a boat cold, its just like any other car engine. So, why isn’t boat oil 5w-40, or 10w-40. Why is it 25w-40? Want a technical reason. Not cause its warm out. Its a simple question. You fancy oil guys don’t know the answer.

The winter rating is a performance metric not a “cold viscosity”. And it’s temperature dependent. No oil thickens as it warms up and goes from a 25 to a 40.

You’re really digging a deep one here. Keep digging.

 

[Cousineddy]

The viscosity units of measure are different at the W than the high number. Oil is always thicker when its cold despite the lower number. The question is, why would you ever want it that thick cold?

 

[kschachn]

The viscosity units of measure are different at the W than the high number. Oil is always thicker when its cold despite the lower number. The question is, why would you ever want it that thick cold?

How cold are you referring to? That’s why I said it was temperature dependent. If you are above freezing then you aren’t talking about much of a difference between any of the winter ratings. It’s still a 40-grade oil and that dominates above where a winter rating makes a difference.

 

[Cousineddy]

Referring to a cold start. Doesn’t matter what temp. A 10W will be thinner than a 25W. If quick silver is a straight 40 weight, then this is not the best oil for your boat. This is basically an oil with a crappy viscosity index. Its 50s oil technology. Oil companies invested tons of engineering to improve VI. So, high VI is better. Im saying a 5w-40 is better for your boat engine. Even 15W-50 is better than the recommended 25w-40. Am I missing anything here?

 

[OVERKILL]

Referring to a cold start. Doesn’t matter what temp. A 10W will be thinner than a 25W. If quick silver is a straight 40 weight, then this is not the best oil for your boat. This is basically an oil with a crappy viscosity index. Its 50s oil technology. Oil companies invested tons of engineering to improve VI. So, high VI is better. Im saying a 5w-40 is better for your boat engine. Even 15W-50 is better than the recommended 25w-40. Am I missing anything here?

Marine engine oil specs have typically been well behind the curve of land-going vehicles. While the current crop of marine inboards from many of the majors spec current automotive oils, that was historically not the case because automotive oils, particularly those produced for this side of the pond, weren't tested for shear resistance/viscosity retention and would not hold up to the usage profile.

There was a period where some engines would spec an HDEO like a 15w-40 or 10w-30.

Our PCM GT40 spec's a 20w-40 for example. I run a 5w-40 HDEO in it (Delvac 1).

The marine usage profile (high load, high RPM) most closely matches that of OTR trucks and industrial equipment (gensets, pumps...etc) which spec HDEO's typically. Using atypical viscosities allowed OEM's to both develop a revenue stream and ensure that engines weren't going to be damaged by the use of automotive lubricants. Could they have just spec'd an HDEO? I'd say in most instances, yes.

As has been noted by other users like @kschachn the Winter rating of the lubricant isn't super important with marine engines because you aren't going to be operating them in freezing temperatures. That an oil is tested to not impact cranking speed at -30C (5w-xx) isn't particularly germane to the operation of an engine that is typically started when it's 20C.

 

[kschachn]

Referring to a cold start. Doesn’t matter what temp. A 10W will be thinner than a 25W. If quick silver is a straight 40 weight, then this is not the best oil for your boat. This is basically an oil with a crappy viscosity index. Its 50s oil technology. Oil companies invested tons of engineering to improve VI. So, high VI is better. Im saying a 5w-40 is better for your boat engine. Even 15W-50 is better than the recommended 25w-40. Am I missing anything here?

Nah you finally got to the point you’ve been trying to make which is bashing the brand. I figured that’s where you were headed but at least we are there now.

And no, the temperature does matter. And besides, as long as the winter rating is appropriate for the starting conditions (which it will be) and the boat starts then the oil only stays at that viscosity for an instant. It immediately begins to warm up from that point, so really it only matters for cranking. Any winter grade will work fine in your application.

Viscosity index is relatively unimportant here. A sufficient MOFT is important, however.

 

[Cousineddy]

Then back to my point…why dont we run 25w-40 in our cars in the summer? Its only at 25 for an instant ya know. You would have your MOFT in your car too.

 

[kschachn]

Then back to my point…why dont we run 25w-40 in our cars in the summer? Its only at 25 for an instant ya know. You would have your MOFT in your car too.

You most certainly could.

 

[dnewton3]

I see this topic as akin to several other lube threads regarding non-street use vehicles.

ATVs are similar in topic; for example Polaris specs a "20w-40" oil for many of their 4 cycle engines. How convenient that the motorsports industry (of which they are a part of) offers a few choices (all OEM branded and/or expensive market specific choices) specs 20w-40, but 15w-40 isn't spec'd.
The marine engine industry is similar; they spec grades a tad bit off "normal" so that they can get a niche reponse in sales. That's my opinion. Also, while marine engines often run at or near WOT with heavy loads, they actually run cooler in their temps (the t-stats are around 160F or so?); this is probably because they source cool lake water, and so to hold down thermal shock risks, they don't run 210F like car engines. I cannot tell you this is all the reason, but it makes sense that it's a portion of the reason. Oils with 25w-40 have fewer VIIs than a 5w-40, and under heavy loads they may well shear less with the more narrow grade rating. Further, there's no EPA fuel economy mandate for marine engines; hence the trend to thinner lubes hasn't hit that market. I do, however, see that many marine motor OEMs often tout fuel effiency in marketing hype now, but the oil vis topic hasn't caught up in that regard, for now.

I don't think you can say it's just one reason; it's probably a combination of these reasons, and others that elude me I'm not even aware of.

 

[OVERKILL]

Then back to my point…why dont we run 25w-40 in our cars in the summer? Its only at 25 for an instant ya know. You would have your MOFT in your car too.

Car manuals used to recommend a broad spectrum of viscosities depending on ambient conditions. We got away from that for simplification and then OEM approvals became such that it didn't matter anyways.

 

[Big E]

I think 25w40 is a Mercruiser Thing.

My Volvo Penta specs 10w40. It's a direct injected VVT 5.3l with catalytic converters.

My 92 OMC 5.7L called for straight 30w IIRC. I used 15w40 until I sold it 25 years later.