Boat Maintenance & Repair Forum

best oil

by betty boop » 15 Feb 2018, 19:57

just looking around for oils as usual its confusing, 4.3 mpi keypart say SAE 30 fully synth - repower selling 15w40 semi synth half the price. what else is there and why not? Ive heard some of the MPI/EFI set off alarms if the oil visco is less than expected in the sensor - wonder if that true. oil is oil right ?
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by Ianfs » 16 Feb 2018, 23:15

I'm sorry that no one has answered you thus far Ian , it seems that the forum goes very quiet, in fact I have been thinking of shutting it down because of this.

However, sometimes it surprises me and it awakens again, so we'll see how it goeas. :D

I happen to think this is an excellent question and one that has been previously debated World wide but for me has not yet found an effective solution.

Consequently I have always used the manufacture recommended oil, which I believe is Quicksilver 25w 40 "Synthetic" for the MPi and "Non synthetic" for previous versions and then searching around for the best prices. Luckily around here I have found a local shop which is a Chandlers but more of an old school boat shop who seem to enjoy having a bit of a deal where oils and supplements can be bought .
Now interestingly enough, last year I had a small issue with a tiny water intrusion in the oil of my little boat which has a 4.3 V6 Carb version. So I drained and filled it several times after finding the problem, but rather than using very expensive marine oil to flush it completely, I used cheapest of cheap oil from the local Motorists Centre to flush it through. They are amazing, a bit of an Aladdins cave for stuff that "Halfrauds" would charge the Earth for. The cheaper oil I used was a 5w 40 oil which you would think was fine, but when fired up, to run the oil through, the engine didn't run as it should. It was OK, but not as crisp as it ought to be and I thought there was damage beyond a simple oil issue.

Cutting a long story short, when I eventually put the recommended oil in, the engine ran like a dream and compressions came back to normal.

I'm not an engineer and can only advise you from a few experience's, but even though I dislike the "hole in the water in which you throw money" or, "I have boat stamped on my forhead, therefore over charge me" theory, I think that because we leave our very very expensive boat engines idle for most or the year, an oil change to save maybe £10-20 is not as bad as we first think. If on the other hand someone else has a great solution, I absolutely would adore to hear it. :)
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by shibbs » 17 Feb 2018, 19:29

It is an interesting question, and I certainly can’t give an answer that will help as I don’t know the differences myself..
, but, it does make me think.
I generally use the recommended oil in the boat engine. For no other reason than exactly what Ian highlights, for the sake of saving a £ or 2 why not.
But is there really any difference?
Why would the oil type make any difference to the compression? I don’t understand why it would be connected?
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by Ianfs » 18 Feb 2018, 10:44

shibbs wrote:It is an interesting question,
But is there really any difference?
Why would the oil type make any difference to the compression? I don’t understand why it would be connected?


So here's what I think, don't know if I'm right or wrong though and I must stress for the sake of space is a very simplistic view.

Boat engines are not started every day, let's face it, some sit around for months, dormant. Cars on the other hand are generally used daily.
I think car engines run at 90 to 99c , boat engines especially raw water engines run much lower around 80c .
Oil ratings numbers relate to thickness and stability I.e. 25w is the Winter rating and thickness and the 40 how the oil performed at running temp.
Now here's the thing, cars are often used in very cold temperatures and need a thinner oil to get to the top quickly, boats are rarely used at low temperatures.
Consequently I think boat oils for Mercruiser and Volvo petrol engines are thicker than car oils and even though there are 12 million boats in the US compared to cars that's not a lot therefore oils made for boats will be made in smaller quantities keeping tbe price higher than car oils.
This is probably all bo**ocks but it seems logical.
So car oils are thinner than boat oils and are cheaper cos of economies of scale.
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by ian h » 18 Feb 2018, 20:51

Not got a real clue but from various forums the thing that does stand outs

Always use the recommended oil. are you really saving that much if the engine is not running as it should do.
Are you doing and damage?

For the sake of a few extra £'s I had always used as per manual . Not worth worrying about, In the scheme of boating the oil is a very small bill.

My engines take 11 litres each. so yes I could make savings BUT at what long term cost .....................
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by betty boop » 21 Feb 2018, 13:58

Thanks all and I an I hear what you said on the first reply, may be a sign of the time but would be missed.

The oil issue hasn't really gone far yet. one of those I got confused and buried my head in the sand moments. Originally it was the Synthetic vs fully synth which got me going. A new player (whos name is gone now) to the UK boat oil market said on their promo material they would never recommend fully synth in a marine engine but then Merc do so but Sierra sell 4.3mpi spec'd merc oil as 1/2 and 1/2 - it makes you wonder who's right or wrong. For the most part I agree saving £10 is nothing and sticking to Sierra for me is fine, but I have little usage in the year compared to some and trailer not sit in raw water coldness for 8 months year. Now if it was a car NO WAY would I/DO I buy £60 5l oil from castro when car parts4 less sell spec'd oil for 1/2 that - as I take 7 litres in the motor that's a BIG saving. An engineer did tell me that oil is oil 15w40 is the same if you spend £10 or £50 a litre. Logically thinking Mercrusier did not reinvent the wheel when they made the 4.3mpi they simple could not afford to re format oil for low volume boat sales, the same way their 10mm oil seal is £5 but the same nitrile washer on ebay is £1. the debate rages on. :lol:
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by Ianfs » 26 Feb 2018, 11:42

Thanks all and I an I hear what you said on the first reply, may be a sign of the time but would be missed.


Thanks Ian, I made a promise that this new forum will be kept going no matter what, it's even in my will, so as long as its being used, it'll be kept going regardless. However, if its not being used there is no point in just having it sit around, except all the info and posts would be kept safe however.

Things seem to have gone back to normal now and maybe the lack of posts was just a blip due to Christmas and horrid Wintery stuff.

Back to your post. If you can remember who the new player in the oil market was I'd be interested to know.

I have noticed Morris oils are making boat engine oil now, or have they been doing it for a while? I'm not sure I agree that engineer that oil is oil no matter which one you buy but I do think that the very expensive one's are a bit of a rip off. I tend to use Comma oil for the Land Rover both engine and gear box seem good.

I do have to agree with you that a 10mm seal at £5 is way over the top and to be fair even at £1 its expensive. :D As you say the debate goes on. :lol:
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