Why oils lose viscosity with use

[oilman]

Viscosity Index Improvers.

An oils viscosity will decrease as the engine temperature rises. Viscosity Index Improvers are added to reduce this thinning. They are a key addative in the production of multigrade oils.

VI Improvers are heat sensitive long chain, high molecular weight polymers that minimise the viscosity loss of the oil at high temperatures. They work like springs, coiled at low temperatures and uncoiling at high temperatures. This makes the molecules larger (at high temps) which increases internal resistance within the thinning oil. They in effect "fight back" against the viscosity loss in the oil.

"Shearing"

The long chain molecules in VI Improvers are prone to "shearing" with use which reduces their ability to prevent the oil from losing viscosity. This "shearing" occurs when shear stress ruptures the long chain molecules and converts them to shorter, lower weight molecules. The shorter, lower weight molecules offer less resistance to flow and their ability to maintain viscosity is reduced.

This shearing not only reduces the viscosity of the oil but can cause piston ring sticking (due to deposits), increased oil consumption and increased engine wear.

Like basestock quality, VI Improvers also vary in quality. As with many items the more you pay, the better the finished article and more expensive, usually synthetic oils are likely to incorporate better VI improvers. All other things being equal the less VI improver an oil contains, the better it will stay in grade by resisting viscosity loss.

Which oils require more VI Improvers?

There are two scenarios where large amounts of these polymers are required as a rule.

Firstly in "wide viscosity span" multigrades. By this I mean that the difference between the lower "W" number and the higher number is large for example 5w-50 (diff 45) and 10w-60 (diff 50) are what is termed as "wide viscosity span" oils.

Narrow viscosity oils like 0w-30 (diff 30) or 5w-40 (diff 35) require far less VI Improvers and therefore are less prone to "shearing".

Secondly, mineral and hydrocracked (petroleum synthetic oils) require more VI Improvers than proper PAO/Ester (Group IV or V) synthetic oils as they have a higher inherent VI to begin with, this is due to differences in the molecular structure of the synthetic base oils compared to mineral oils.

It is a fact that many synthetics require significantly less VI Improver to work as a multigrade and are therefore less prone to viscosty loss by shearing.

Cheers
Simon

 

[996jimbo]

More top info!

Could you give us a quick appraisal on the position regards mixing of different types of oil. If one has a service and forgets to check what sort of oil they have what are the possible consequences of topping up with synthetic when it was serviced with semi, or mineral? And where does Magnatec fall in all of this?

Many thanks,

Jim

 

[oilman]

Oils mix without a problem but you have to remember one thing.

By mixing a semi with a fully you will not get the full benefits of the fully and therefore you may be wasting your money paying for it.

Magnatec is a mineral basestock with small % of ester added to give it polarity (the clings to your engine claim) as ester molecules are polar.

It is a different proposition to a proper synthetic oil which contains 80% poly alpha olefins (ala Mobil1) and 20% esters.

Esters

All jet engines are lubricated with synthetic esters, and have been for 50 years, but these expensive fluids only started to appear in petrol engine oils about 20 years ago.

Thanks to their aviation origins, the types suitable for lubricants (esters also appear in perfumes; they are different!) work well from ?50 degC to 200 degC, and they have a useful extra trick.

Due to their structure, ester molecules are ?polar?; they stick to metal surfaces using electrostatic forces. This means that a protective layer is there at all times, even during that crucial start-up period. This helps to protect cams, gears, piston rings and valve train components, where lubrication is ?boundary? rather than ?hydrodynamic?, i.e. a very thin non-pressure fed film has to hold the surface apart. Even crank bearings benefit at starts, stops or when extreme shock loads upset the ?hydrodynamic? film.

The blending tricks (mainly cheapening) that go on in the oil industry are vary varied but to give some explanation, here's a description of basestock qualities. Magnatec is Group II with a dash of Group V. A top synthetic oil is either 100% Group IV or a blend of Group IV and V which is the best quality.

Basestock categories and descriptions

All oils are comprised of basestocks and additives. Basestocks make up the majority of the finished product and represent between 75-95%.

Not all basestocks are derived from petroleum, in fact the better quality ones are synthetics made in laboratories by chemists specifically designed for the application for which they are intended.

Basestocks are classified in 5 Groups as follows:

Group I

These are derived from petroleum and are the least refined. These are used in a small amount of automotive oils where the applications are not demanding.

Group II

These are derived from petroleum and are mainly used in mineral automotive oils. Their performance is acceptable with regards to wear, thermal stability and oxidation stability but not so good at lower temperatures.

Group III

These are derived from petroleum but are the most refined of the mineral oil basestocks. They are not chemically engineered like synthetics but offer the highest level of performance of all the petroleum basestocks. They are also known as ?hydrocracked? or ?molecularly modified? basestocks.
They are usually labelled/marketed as synthetic or semi-synthetic oils and make up a very high percentage of the oils retailed today.

Group IV

These are polyalphaolefins known as PAO and are chemically manufactured rather than being dug out of the ground. These basestocks have excellent stability in both hot and cold temperatures and give superior protection due to their uniform molecules.

Group V

These special basestocks are also chemically engineered but are not PAO.
The main types used in automotive oils are diesters and polyolesters. Like the group IV basestocks they have uniform molecules and give superior performance and protection over petroleum basestocks. These special stocks are used in all aviation engines due to their stability and durability. Esters are also polar (electro statically attracted to metal surfaces) which has great benefits. They are usually blended with Group IV stocks rather than being used exclusively.

It is common practice for oil companies to blend different basestocks to achieve a certain specification, performance or cost. The blending of group IV and V produces lubricants with the best overall performance which cannot be matched by any of the petroleum basestock groups.

Cheers
Simon

 

[996jimbo]

I always understood that to top up with synthetic in to non-synthetic would be a BAD THING. Is it just a no better thing then, ie won't result in any ill effects?

So fully synthetic oils all have the "magnetic" charateristic of Magnatec anyway (dwelling on the marketing basis of Magnatec) but as synthetic go on to perform better?

 

[oilman]

"Proper" synthetic oils are superior, Magnatec apes the benefits of polarity.

Proper synthetics are obviously more expensive and used in applications from road to race.

Cheers
Simon

 

[996jimbo]

Thanks very much for that.

I've never actually bought Magnatec - now reassured to know that it doesn't offer anythig I don't already get from synthetic stuff.