Drawbacks of Viscosity Improving additives
Multi-grade motor oils perform a great service not being too thick at cold startup to prevent engine wear by providing more instantaneous oil flow to critical engine parts. However, there is a draw back. These additives shear back in high heat or during high shear force operation and break down causing some sludging. What's worse is once the additive begins to be depleted the motor oil no long resists thinning so now you have a thinner motor oil at 99 degrees celsius (210 degrees F).
Multi-grade motor oils perform a great service not being too thick at cold startup to prevent engine wear by providing more instantaneous oil flow to critical engine parts. However, there is a draw back. These additives shear back in high heat or during high shear force operation and break down causing some sludging. What's worse is once the additive begins to be depleted the motor oil no long resists thinning so now you have a thinner motor oil at 99 degrees celsius (210 degrees F).
Your 10W-30 motor oil can easily become a 10W-20 or even a SAE 10 (10W-10) motor oil.
I don't have to tell you why that is bad. The more VI additives the worse the problem which is why auto manufacturers decided to steer car owners away from motor oils loaded with VI additives like the 10W-40 and 20W-50 viscosities.
The less change a motor oil has from high to low temperatures gives it a high Viscosity Index. Synthetic motor oils that are made from Group IV (4) PAO base stocks have Viscosity Indexes of more than 150 because they are manufactured to be a lubricant and don't have the paraffin that causes the thickening as they cool. But petroleum based motor oils (Group I (1) & II (2)) usually have Viscosity Indexes of less than 140 because they tend to thicken more at the colder temperature due to the paraffin despite the addition of Viscosity Improving additives. The higher the Viscosity Index number the less thinning and thickening the motor oil has.
In other words, high number good, low number bad.
Low numbers thicken more as they cool and thin more hot. You see these Viscosity Index ratings posted on data sheets of motor oils provided by the manufacturer.
As already mentioned, VI improving additives can shear back under pressure and high heat conditions leaving the motor oil unable to protect the engine properly under high heat conditions and cause sludging. Also there is a limit to how much viscosity improving additives can be added without affecting the rest of the motor oil's chemistry.
Auto manufacturers have moved away from some motor oils that require a lot of viscosity improving additives, like the 10W-40 and 20W-50 motor oils, to blends that require less viscosity additives like the 5W-20, 5W-30 and 10W-30 motor oils. Because stress loads on multi viscosity motor oils can also cause thinning.
Many racers choose to use a straight weight petroleum racing motor oil or a PAO based Synthetic which do not have the VI additives. But only the Group IV (4) PAO based synthetics generally don't need VI additives. Read on to learn why:
What about synthetic motor oils? Do they need Viscosity Additives?
Group IV (4) and Group V (5) base oil (synthetics) are chemically made from uniform molecules with no paraffin and generally don't need Viscosity Additives. However, in recent years Group III (3) based oils have been labeled "synthetic" through a legal loophole.
Group IV (4) and Group V (5) base oil (synthetics) are chemically made from uniform molecules with no paraffin and generally don't need Viscosity Additives. However, in recent years Group III (3) based oils have been labeled "synthetic" through a legal loophole.
These are petroleum based Group II (2) oils that have had the sulfur refined out making them more pure and longer lasting.
Group III (3) "synthetic" motor oils must employ Viscosity Additives being petroleum based.
Group V (5) based synthetics are usually not compatible with petroleum or petroleum fuels and have poor seal swell. These are used for air compressors, hydraulics, etc.
It's the Group IV (4) PAO based synthetics that make the best motor oils.
They are compatible with petroleum based oils and fuels plus they have better seal swell than petroleum. Typically PAO based motor oils use no Viscosity Index additives yet pass the multi-grade viscosity requirements as a straight weight! This makes them ideal under a greater temperature range. One advantage of not having to employ Viscosity Improving additives is having a more pure undiluted lubricant that can be loaded with more longevity and performance additives to keep the oil cleaner longer with better mileage/horsepower.
How do I know what motor oil is a Group IV (4) based PAO synthetic motor oil?
As more and more large oil companies switched their "synthetic" motor oils to the less expensive/more profitable Group III (3) base stocks it has become much easier to identify which are PAO based true synthetic.
As more and more large oil companies switched their "synthetic" motor oils to the less expensive/more profitable Group III (3) base stocks it has become much easier to identify which are PAO based true synthetic.
Of the large oil companies, only Mobil 1 Extended Performance, as of this writing (12-16-2012), is still a PAO based true synthetic.
The rest, including regular Mobil 1 and Castrol Edge have switched to the cheaper/more profitable Group III (3) petroleum based "synthetic" motor oil. AMSOIL Synthetic Motor Oils are PAO based true synthetic motor oils with the exception of the short oil drain OE and XL synthetic motor oils sold at some Auto Parts Stores and Quick Oil Change Centers. This leaves more than 20 PAO based true synthetic motor oils manufactured and marketed by AMSOIL with only a few Group III (3) based synthetic motor oils identified by the "OE" and "XL" product name.
So as you can see, the average performance of motor oils can be affected by how they change during their service life. Multi grade petroleum can lose viscosity and thin causing accelerated wear as the VI additives shear back. Straight weight petroleum (i.e. SAE 30, SAE 40) thicken a lot as they cool meaning longer time before lubricant reaches critical parts on cold starts, but have no VI additives so they resists thinning. However, they can degrade and thicken as heat and by products of combustion affect the unsaturated chemistry. Group III (3) synthetics resists this degradation much better, but being petroleum based employ some VI additives which is a negative and typically don't have as good performance in the volatility viscosity retention areas. Only the Group IV (4) PAO base synthetics have the saturated chemistry to resist degrading when exposed to the by products of combustion and heat, plus typically employ no VI additives making them very thermally stable for longer periods. For this reason the Group IV (4) synthetics maintain peak mileage and power throughout their service life
Want to find out more? Read on from here : http://www.upmpg.com/tech_articles/motoroil_viscosity/
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