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Old 21 November 2017, 14:30   #1
eze
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4 stroke fully synthetic engine oil - auto or marine?

Somehow I feel you guys have done this one before, So for the benefit of us folk with less knowledge can we open the debate once again.

It is possible to obtain 20 Litres of a fully synthetic 5w/40 for circa 50-60 that is auto grade. However, if we want Marine grade oil the price is seemingly double or treble the above price.

Question;

Is it okay to use a fully synthetic 5w/40 auto engine oil in a Marine outboard and if not then why not?

thanks
eze
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Old 21 November 2017, 14:51   #2
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Suzuki "recommend" their own oil, but concede that you can use any good quality oil as long as it meets or exceeds the api/sae rating of theirs.
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Old 21 November 2017, 14:57   #3
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There are a handful of differences I'm aware of.

1. If you are in warranty you better use their oil.
2. Foam inhibitors due to higher sustained rpm.
3. Corrosion inhibitors.
4. Price obviously.

Lots of people run correct spec car oil in their outboards though. in 25 years of boating I've never put car oil in my outboards or inboards so no first hand experience.
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Old 21 November 2017, 16:48   #4
eze
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[QUOTE=dubrus;762346]There are a handful of differences I'm aware of.

1. If you are in warranty you better use their oil.
2. Foam inhibitors due to higher sustained rpm.
3. Corrosion inhibitors.
4. Price obviously.

QUOTE]


1 - agreed, best use their oil.

2 - Sounds reasonable tbh, but 4s motorbikes will run 3-4 k rpm on cruise and also go up to 12k rpm full chat. but I get your point.

3 - Is there any difference here between auto & marine as both are by design internally sealed?

4 - agreed.


Thanks for the replies chaps.
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Old 21 November 2017, 16:57   #5
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Give millers oils a ring they will tell you all you need to know.
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Old 21 November 2017, 17:01   #6
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The difference is probably down to moisture build up, normally cars won't build up much unless you take them into the lake!

The reason the marine oil costs more is simply down to the market size and the extra additives so volume discounting is way less than road vehicles....plus boat doesn't stand for Break Out Another Thousand for nothing. That doesn't mean car oil won't work of course, that is upto the owner to decide.
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Old 21 November 2017, 18:56   #7
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If the spec matches then just use it I doubt the oil knows wether it's in a boat or car bike or truck
I've run standard 15 40 in my inboards for years and wouldn't even consider buying branded marine oil
Obviously the manufacturers want you to believe it's different but they would there making a tidy profit on their own brand oil
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Old 22 November 2017, 05:27   #8
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As Dubrus says in post #3... there are big differences in the operating conditions of an outboard at sea to a car and marine oil is different with particular regard to constant high revs, vibration from the OB, movement from sea state, sometimes infrequent use and corrosion from sea air.

With the small amount of oil used in the OBs I've owned up to 20hp the extra cost of marine spec oil is insignificant... but even with a larger OB I'd still treat it.

Which brand of marine oil used is a different choice and a personal thing.

Inboard marine diesels may be a different matter. I only have experience of these from the 1960/70s when we used car oil.
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Old 22 November 2017, 06:00   #9
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Old 22 November 2017, 08:13   #10
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Quote:
Originally Posted by Fenlander View Post
As Dubrus says in post #3... there are big differences in the operating conditions of an outboard at sea to a car and marine oil is different with particular regard to constant high revs, vibration from the OB, movement from sea state, sometimes infrequent use and corrosion from sea air.

With the small amount of oil used in the OBs I've owned up to 20hp the extra cost of marine spec oil is insignificant... but even with a larger OB I'd still treat it.

Which brand of marine oil used is a different choice and a personal thing.

Inboard marine diesels may be a different matter. I only have experience of these from the 1960/70s when we used car oil.
Maybe a 20hp uses only a small amount but it takes close on 30l to do my two yanmars
With regards to the differences between automotive and marine, a 4 stroke outboard is working virtually like a dry sump engine which all but does away with the oil splash or boat roll problem, all the oil is delivered by the pump then migrates back to the sump which is well below where the crank will contact the oil. A bike engine works far harder than an outboard,the hp per cc is way higher so stress isn't as high on an outboard as a bike engine,boat engines also do way less hours than their on road counterparts
As for corrosion the sump system is fully sealed with complex breather systems so corrosion shouldn't be an issue in normal use
The only benefit in using branded oil in my opinion is the placebo effect that you've used the 'correct oil' however in my opinion any oil within spec is fine.
If anyone can provide indisputable proof that marine branded oils provide any benefit over alternative same spec oils I'm all ears
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