- Blue smoke (oil getting past valve seats/piston rings)
- High oil loss
- Regular track usage
Engine oil is composed of an additive package, a viscosity modifier (to reduce the amount the oil thins at high temperatures) and usually two or more base stocks. These base stocks come in different grades:
Group I - these are the least refined, they have high sulfur and permanently thicken the quickest.
Group II - these have been refined to remove some sulfur, however are still relatively low quality, not used much in Europe
Group III - these have been significantly refined to remove sulfur and reduce high temperature oxidation (causing thickening). In some places these are referred to as synthetic due to the number of chemical processes undergone to produce the base stock.
Group IV - these are synthetically made and are tightly controlled, they have minimal sulfur and are very oxidatively stable.
Group V - this group refers to everything else available as a base stock, such as esters. They tend to be expensive and with properties similar to that of group IV.
Mineral oils are just a mix of different length hydrocarbons, as the refinement process gets better, the molecular weight distribution (the number of different lengths) reduces, this makes the base stock properties more predictable.
As you may have guessed from reading this, the price for the base stock increases as the group increases. Companies want to price optimise their products so put as much of the lowest quality material they can in. They combine multiple base stocks to target the correct viscosity (in simple terms, if you want a viscosity of 5, and have only base stocks of 4 and 6, you combine both at equal ratios to get the target viscosity). This graph illustrates the resulting situation.
However, there are factors stopping low quality base stocks being used, as oils get thinner, more of the thin base stock molecules get boiled off as the oil is heated, this means great viscosity increase (as without the thin material, the oil gets thicker) and regular top ups, here - everything marked in red is lost, this amount of loss in unacceptable and will fail engine tests (for requiring too many top ups, and resulting in large viscosity increase as the thin base oil is constantly being lost), as well as bench criteria (the oil is heated overnight and weight measured, high loss is a fail). Companies solve this by switching to a higher quality base stock, the overaIl viscosity stays the same, but the amount lost when heated reduces.
direct comparison:
The problem is the viscosity requirements don't change much with viscosity grade, so if the company wanted to produce a higher viscosity grade oil (in this case go from a 5W-3 to a 15W-50 oil), they can switch back to the poor quality base stock without much concern (before they were targeting a viscosity of 5 with base stocks of viscosity 4 and 6, now they are targeting a viscosity of 7 with base stocks 6 and 8). This means that in practice, oil quality has potential to get worse when moving to a higher viscosity grade as the limits don't move, so allow for worse base stocks.
If you want to ensure you have good quality base stocks (which will limit oil loss through evaporation), then look for specifications which tighten the oil loss limit. Daimler MB229.5 has a limit of 10 % respectively compared to the 13 % as an industry standard (effectively moving the red line on the graphs to the left).
I hope this helps.
Edited by fezzasus, 12 April 2013 - 12:04 PM.
This is what you were trying to explains to me the other day 
Thanks all the same for actually taking the time 
