Mineral / Semi-Synthetic / Synthetic Oil ..... Viscosity Selection

JumbleDuck said:
Does this help?
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Only a bit. Nice to see some actual viscosity data but most of the temperature range is irrelevant. Kinematic viscosity includes the density of the fluid but here we are considering oils all of essentially the same density so better to stick to dynamic viscosity only.
1. It gives data for an irrelevant range of temperatures
2. It introduces the concept of kinematic viscosity which acknowledges and includes the density of the fluid. All the oils we are considering here are of essentially the same density and with very little variation of density between 0 and 100 C? Stick with dynamic viscosity.
 
On my previous boat, I run my 50 year old Volvo Penta MD11C on full synthetic for many many years simply because synthetic oils are superior in every way.
I now run my BMC Thornycroft on full synthetic for the same reason. Older engine deserve to be looked after using the best up to date lubricant technology. I choose the oil viscosity specified for that engine or equivalent for synthetic.
 
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penberth3 said:
To be absolutely clear, the "20" isn't the viscosity is it? It's a grade number - and as you explain, the viscosity of the "20" or whatever will vary with temperature.
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Indeed so. The viscosity of every liquid reduces with increasing temperature which is exactly why a 15W oil has the same viscosity as a 10W oil but the 15W oil has to be at a slightly higher temperature .... which is why my theory is that it is relatively "safe" to switch downwards at the "Winter" end and upwards at the "Hot" end but not the reverse.

Richard
 
reeac said:
Only a bit. Nice to see some actual viscosity data but most of the temperature range is irrelevant. Kinematic viscosity includes the density of the fluid but here we are considering oils all of essentially the same density so better to stick to dynamic viscosity only.
1. It gives data for an irrelevant range of temperatures
2. It introduces the concept of kinematic viscosity which acknowledges and includes the density of the fluid. All the oils we are considering here are of essentially the same density and with very little variation of density between 0 and 100 C? Stick with dynamic viscosity.
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I didn't write the specs, which I agree are remarkably complicated.
 
RichardS said:
I've no idea how you can possibly decide from my OP that I don't fully realise that, but there you go.
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Because you keep referring to "15W" viscosity or the "10W" viscosity.

RichardS said:
I've never said that 15W grade becomes a 10W grade. I have consistently referred to the 15W viscosity or the 10W viscosity. Please read the OP again.
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If you mean "the viscosity of a 15W oil" you would be better to do say so, though note that the grade only defines a range and that different 15W oil can have different viscosities.
 
I often considered using synthetic oil, however after consulting some of the oil companies technical help lines, none have recommended synthetic, all have recommended various mineral or semi synthetic oils. This is for a Perkins sabre 135 plus about 10 different ex military vehicles from 25 to 60 year old. Not one recommendation from oil chemists in the industry, that’s enough for me to decide.
 
JumbleDuck said:
Because you keep referring to "15W" viscosity or the "10W" viscosity

If you mean "the viscosity of a 15W oil" you would be better to do so, though note that the grade only defines a range and that different 15W oil can have different viscosities.
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We're only referring to oil here, so when I say 15W, it is surely understood that that means "15W oil".

Your point therefore boils down to whether my "15W oil viscosity" is any more meaningful than your "viscosity of a 15W oil". :unsure:

It is indeed a viscosity range for each SAE rating but that does not impact upon the point being discussed. However, please feel free to substitute "viscosity range" whenever I say "viscosity" if you are happier with that.

Now, please can we stop this discussion on such obvious terminology and concentrate on the actual point under discussion?

Richard
 
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Bran said:
I often considered using synthetic oil, however after consulting some of the oil companies technical help lines, none have recommended synthetic, all have recommended various mineral or semi synthetic oils. This is for a Perkins sabre 135 plus about 10 different ex military vehicles from 25 to 60 year old. Not one recommendation from oil chemists in the industry, that’s enough for me to decide.
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None are going to because your engine has not been tested by the manufacturer using a synthetic oil. If you changed to synthetic and your engine then seized, which would be for totally unrelated reasons apart from one potential issue with synthetic oils, then whoever gave you that advice could potentially face legal action. Why would they take the risk?

Richard
 
Bran said:
I often considered using synthetic oil, however after consulting some of the oil companies technical help lines, none have recommended synthetic, all have recommended various mineral or semi synthetic oils. This is for a Perkins sabre 135 plus about 10 different ex military vehicles from 25 to 60 year old. Not one recommendation from oil chemists in the industry, that’s enough for me to decide.
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The industry will not change the specified/recommended oil that was identified suitable for engines already designed/produced. New engine designs will certainly undertake tests using synthetic oils and therefore will specify the appropriate synthetic oil for that engine.
 
Some o the conversations I had were not official responses as I was installing equipment in the laboratory at the time so no liability issues, the reason they gave was invariably that there were to many variables to be sure what the benefits or risks would be. They did however recommend oils that were of different grades to those recommended by the original engine manufacturers.
One official recommendation is from the Shell online oil checker for my boat engine, it comes up with one grade lighter than Perkins recommended, so I use that.
I was keen to go the synthetic route and was asking what grade to use, I could never get any of the technical staff to come up with an answer.
No reason that I shouldn’t get less than 50 to 100,000 hours using the recommended oil was one comment.
Interestingly I met someone who worked at Perkins sabre setting up and testing the engines, I was the first actual user of one of those engines he had ever met!
Also interesting that one chemist I was working with mentioned that with some new vehicle engines the engines and data are supplied to the oil companies and they have to develop a suitable oil for use. Probably explains why there are so many different engine oils for modern engines. Not relevant to us of course, it just explains that some modern engines are pushing engine development to the limit that oil suppliers have to add small amounts of additives to prevent some engine issues, that’s what pushes the price up of some oils.
 
Bran said:
Some o the conversations I had were not official responses as I was installing equipment in the laboratory at the time so no liability issues, the reason they gave was invariably that there were to many variables to be sure what the benefits or risks would be. They did however recommend oils that were of different grades to those recommended by the original engine manufacturers.
One official recommendation is from the Shell online oil checker for my boat engine, it comes up with one grade lighter than Perkins recommended, so I use that.
I was keen to go the synthetic route and was asking what grade to use, I could never get any of the technical staff to come up with an answer.
No reason that I shouldn’t get less than 50 to 100,000 hours using the recommended oil was one comment.
Interestingly I met someone who worked at Perkins sabre setting up and testing the engines, I was the first actual user of one of those engines he had ever met!
Also interesting that one chemist I was working with mentioned that with some new vehicle engines the engines and data are supplied to the oil companies and they have to develop a suitable oil for use. Probably explains why there are so many different engine oils for modern engines. Not relevant to us of course, it just explains that some modern engines are pushing engine development to the limit that oil suppliers have to add small amounts of additives to prevent some engine issues, that’s what pushes the price up of some oils.
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A synthetic oil of a particular SAE viscosity rating is exactly the same as a mineral oil or semi-synthetic oil of the same rating, within the ranges of the specification, of course. If you wish to change to synthetic oil, and, from a technical perspective, it is advantageous to do so from every angle, then I would recommend using exactly the same SAE grade. The only reason not to do so would be because the same grade is not available in synthetic oil. This used to be the case but is no longer so.

I have explained above why it would not be detrimental to use a thinner oil at the the W end of the scale, although I would hesitate to use a thinner oil at the hot end.

Richard
 
JumbleDuck said:
I tried.
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You are still missing the point, I'm afraid.

A 5W oil is has a lower viscosity at zero degrees (or whatever) than an 10W oil. However, if you start to warm up a 10W oil, it will soon reach the same viscosity of the 5W oil, albeit at a higher temperature, so an engine which is using 10W oil will sometimes be running on an oil which is the same viscosity as a 5W oil but, assuming that the oil meets the manufacturers ambient temperature specification, the reverse is not true.

I did suspect that I had developed a novel approach here but I am surprised that others are having so much trouble grasping the concept.

Richard
 
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RichardS said:
You are still missing the point, I'm afraid.

A 5W oil is has a lower viscosity at zero degrees (or whatever) than an 10W oil. However, if you start to warm up a 10W oil, it will soon reach the same viscosity of the 5W oil, albeit at a higher temperature, so an engine which is using 10W oil will sometimes be running on an oil which is the same viscosity as a 5W oil......
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This is wrong. 10W warmed up gets thinner but it's still 10W. 5W warming up also gets thinner - so the two never meet.
 
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