Viscosity Index

[The Critic]

Folks,
What is a viscosity index supposed to tell you?

Is it supposed to give me an idea of which Group (Grp III, IV, etc) that the oil is supposed to fall into?

If so, how can I tell?

Is there a certain viscosity range for Group III oils, or Group IV oils, etc?

Thanks,
Michael

 

[ericgl]

VI is the rate of change in viscosity with temperature. The higher the VI the less the vicosity changes with temperature. Most passenger car motor oils use viscosity index improvers (VIIs) which are polymers that 'relax' and become physically larger when warm, increasing the fluid's apparent viscosity. The use of these VIIs mask the base oils inherent viscosity index and make conclusions impossible.

 

[widman]

It determines what the viscosity will be at temperatures other than those posted on the spec sheet. I just ran charts for someone where the difference in VI between the two oils they were considering caused the following for an oil that is 8000 cSt at 40C
- - - -Oil 1 - - - - Oil 2
20C -- 35000 cSt --- 75,000 cSt
30C -- 16000 cSt --- 24,000 cSt
70C - - 1400 cSt --- 760 cSt

So while both had the same ISO viscosity, one was imposible to pump at 20C and had less film at operating temperatures than the other.

 

[Big Jim]

Viscosity index is a seperate measure from Group number.

The easiest way to tell if an oil has a high viscosity index is to look at the two numbers in the viscosity rating. The "W" number is the "winter" or "cold" rating and is measured when the oil is cold. The second number is the hot rating and is taken when the oil is hot. The specs for the testing actually give precise temperatures as well as precise flow numbers. If the spread between the "W" number and the other number is very big, the oil has a high viscosity index.

Whatever the base oil viscosity index, additives may be needed to bring it to the desired viscosity index. This class of additives are called "viscosity index improvers". Synthetic base oils have a higher viscosity index to begin with and require less viscosity index improver additives than conventional oil.

It is generally felt that it is best to start with a good base oil (synthetic or conventional) and to use only the additives needed to bring it to the spec that you are targeting. The more additives, the less actual lubricating oil. Exactly how much of what is highly debatable.

 

[smthrj53]

Michael,

My read of your question indicates the previous replies may be more technical than you were looking for. Viscosity index is a way of measuring how 'thick' or 'heavy' the oil is. A 40 weight oil is thicker than a 30 weight oil.

The group type is an indication of what chemistry base the oil is made from.

 

[Al]

VI is pretty much of a useless number for the user. This might help:

It came from here:
http://www.chevron.com/prodserv/BaseOils/gf4_faq.shtml
The VI for Esters/PAO's will be greater (probably) but is not specified.

 

[MolaKule]

The VI tells you how stable the oil will be over a temperature range. The higher the VI, the oil will thin out less at high temperatures and thicken less at cold temperatures.

Most GroupIV's have Viscosity Indices of about 130 or greater and some esters (Group V) have indices of 140 to 170 or greater.