I am posting this question for a friend of mine. He is a retired physician and knows more about chemistry than I do.
Why does synthetic oil last longer?
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It depends on which synthetic and which conventional, but, in general;
Syn can last longer because it's generally 'better' at TBN retention, resisting shear, high temp stability, dirt suspension, and all the other metrics that can be used to quantify an oils performance. It's 'custom built' in a lab.
Conventional is kind of a 'close enough' botch job that will do the job but not as well or for as long.
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Don't most synths have a higher starting tbn?
Originally Posted By: aquariuscsm
Don't most synths have a higher starting tbn?
Define which syn base oil or blend because all syn oils are not equal and gp III oils are better refined than gp II oils which cleaner that base 1 oils due to the higher refining levels [if I worded it properly]. This isn't the 1970's oils anymore.
Originally Posted By: Kuato
yep, and one reason it's so much better is that the carbon chain molecules are all uniform in length. In a non-synthetic you have various lengths; the shorter ones can burn off more easily, the longer ones are thicker and can cause sludge.
Synthetic oil has better marketing. Type SN oil will be virtually indistinguishable from synthetics over the life of a motor. And it better be, because its three times as expensive as Type SG used to be, and costs what synthetic used to cost!
The difference between synthetic oil and conventional oil is the base oil. Additive packages are the same.
Primarily, the reason synthetic oil lasts longer is because its base oil oxidizes (cooks) slower. However, there are different synthetic base oils with different oxidation resistance.
Secondarily and much less importantly, synthetic oils tend to have less viscosity-index-improver (VII) polymers, which helps reduce permanent viscosity loss in used oil as well as polymer contribution to dirt and sludge, which also helps increase the oil-change interval.
Yes as Gokhan says, a synthetic base oil be it Group 3 or Group 4, offers better oxidation resistance than mineral base (Group 1 or Group 2) oil. Not that many oils use Group 1 anymore.
Also synthetics have a naturally higher viscosity index (VI) so less polymer VII (Viscosity index improver) is required and so there is less to shear or break down and form deposits. Given the same grade of oil, eg 5W30.
Most synthetics tend to have a better add pack, but this more marketing (put all the good stuff together) but obviously this isn't strictly required and there will be exceptions.
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Originally Posted By: FordCapriDriver
Originally Posted By: Kuato
yep, and one reason it's so much better is that the carbon chain molecules are all uniform in length. In a non-synthetic you have various lengths; the shorter ones can burn off more easily, the longer ones are thicker and can cause sludge.
Originally Posted By: L_Sludger
Synthetic oil has better marketing. Type SN oil will be virtually indistinguishable from synthetics over the life of a motor. And it better be, because its three times as expensive as Type SG used to be, and costs what synthetic used to cost!
My understanding is that Synthetic oils resist shear more, better cold flow characteristics, and can take the heat better. Which is why they are very advantageous in a air cool shared engine/transmission motorcycle.
I think there is more base stock in synthetic oil, (Silkoline pro comp) and the fact that "true synthetic" is made in a lab with more uniformed molecules.
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Originally Posted By: FordCapriDriver
Originally Posted By: Kuato
yep, and one reason it's so much better is that the carbon chain molecules are all uniform in length. In a non-synthetic you have various lengths; the shorter ones can burn off more easily, the longer ones are thicker and can cause sludge.
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Magic
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From my experience with vehicle ownership and mechanics being a strong hobby I have not seen any vehicle manufacturers state that it is acceptable to extend drain intervals based on the use of a synthetic oil. The only industry promoting it is the manufacturer of the oil which, incidentally is not the warrantor of the engine.
Use the specification that is called for in the application. The marketing term "synthetic" guarantees you NOTHING.
It is supposed to hold up better to heat and oxidation than conventional oil. It is also supposed to come from a better base stock, and contain a better add pack. As ARB1997 said, it's Magic.
I believe Toyota did or still does. Corolla OCI was 5,000 on conventional and 7,500 on syn.
Originally Posted By: FowVay
From my experience with vehicle ownership and mechanics being a strong hobby I have not seen any vehicle manufacturers state that it is acceptable to extend drain intervals based on the use of a synthetic oil. The only industry promoting it is the manufacturer of the oil which, incidentally is not the warrantor of the engine.
Use the specification that is called for in the application. The marketing term "synthetic" guarantees you NOTHING.
My 1975 Ford Capri's owner's manual states that oil changes should be done at 6000 miles or 12 months, that's fairly long for 1975, and on Mineral API SE oils.
The manual for my Sienna says 5K (conventional assumed), but Toyota says I can go 10K if I have synthetic.
But my Civic manual specifically says that the use of synthetic does not allow me to go beyond the 7500 I can go with normal driving.
Originally Posted By: TheOilWizard
I believe Toyota did or still does. Corolla OCI was 5,000 on conventional and 7,500 on syn.
Originally Posted By: FowVay
From my experience with vehicle ownership and mechanics being a strong hobby I have not seen any vehicle manufacturers state that it is acceptable to extend drain intervals based on the use of a synthetic oil. The only industry promoting it is the manufacturer of the oil which, incidentally is not the warrantor of the engine.
Use the specification that is called for in the application. The marketing term "synthetic" guarantees you NOTHING.
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