Why don't we just use a SAE 10 motor oil so we can get instant lubrication on engine start up?
The reason is simple: it would be a SAE 10 motor oil at 210° F! The lower the viscosity, the more wear will inevitably occur. This is why it is best to use the proper oil viscosity recommended by the auto manufacturer as it will protect hot and at cold start ups. Obviously a 10W-10 motor oil won't have the film strength to prevent engine wear at full operating temperature like a 5W-20, 10W-30 or 5W-30 motor oil for example.
The VI additives have the effect of keeping the oil from thinning excessively when heated. The actual mechanics of this system are a little more complex in that these additives are added to a
thinner oil so that it will be fluid at a cold temperature. The VI additives then prevent thinning as the oil is heated so that it now can pass the SAE viscosity rating at 210. For example; if you have a SAE 10 motor oil it will flow like a 10W at the colder temperature. But at 210 degrees it will be a SAE 10 giving us a 10W-10 or SAE 10 viscosity rating. Obviously this is good at cold start up, but terrible at engine operating temperature especially in warmer climates. But by adding the VI additives we can prevent the oil from thinning as it is heated to achieve higher viscosity numbers at 210 degrees. This is how they make a petroleum based motor oil function for the 10W-30 rating. The farther the temperature range, like with a 10W-40, then more VI additives are used. With me so far? Good, now for the bad news.
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 210 degrees. 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.
http://www.upmpg.com/tech_articles/motoroil_viscosity/
