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Oil viscosity losses in the engine

Viscosity of engine oils and oil thickeners



It is known that all engine oils contain additives which improve and maintain different properties. During engine running these oil additives deteriorate under thermal and mechanical loads. Oil molecules also suffer a change. When all these changes are at maximum level, the engine oil must be changed.

One of the main properties which allow defining of oil change deadline is a change in oil viscosity. Engine oil properties depend a lot on this characteristic. Oil viscosity change only on 5%, is warning, and 10% change is a critical level.

It is very important to understand that oil viscosity change doesn’t happen unevenly. This is a progressive process between oil changes. Main causes that lead to oil viscosity change are shown in the table:

General causes of engine oil viscosity change


  Viscosity reduce Viscosity increase
Changes on molecule level

- Thermal oil molecule destruction

- Viscosity modificators (polymers) destruction

- Thermal polymerization of engine oil and additives

- Oil oxidation

- Oil losses on vaporization

- Deposition formation

Changes linked with pollution

- Dilution with fuel

- Ingress of cooling agent

- Dilution with solvents

- Ingress of water

- aeration (mixing with air)

- Ingress of antifreez


Changes linked with oil pollution should be eliminated during diagnostic and repair at car service stations.

The most interesting changes happen on molecule level. They are interesting and can’t be avoided, because they are fundamental. But these changes can be controlled.

Causes which provide oil viscosity increase will be considered in an independent article devoted to antiwear oil properties. Here we will examine reverse process. Here we will show the most probable consequences of oil viscosity decrease:

  • Reduce of oil film thickness on friction surfaces, and redundant wear, sensitivity to contaminations, oil film deformation or rupture under high loads and engine run-up.

  • Increase of friction in engine parts working in different modes (piston rings, gas-distributing unit) will lead to the excessive fuel consumption and heat release.

It is known that standard SAE J300 approves 4 methods for engine oil viscosity identification. Since oil viscosity decrease appear in running engine, the most convenient method of oil viscosity identification will be HTHS test method.

This parameter (High-Temperature High-Shear rate viscosity) is defined in almost real working conditions of engine oil in the unit “piston ring and cylinder surface”. There are similar conditions that occur on surface of camshaft buttons and bearings of the crankshaft at high temperatures and loads. Temperature is +150°C at HTHS test method, and shear rate is 1.6*106 1/sec.

To understand better the last value we can show some life examples, where shear rate has similar parameters:

  • - Painting with a roller with speed 160 km/sec;

  • - Water squeezing from a 10ml syringe with a needle – 1/10 of sec.;

  • - Smearing of 200 000 pieces of bread with butter by 1 person during 1 min.

So, HTHS viscosity is linked with protective oil properties and fuel consumption. The last statement is confirmed by researches (pic.1).

 Pic 1.

Relationship between oil consumption and oil properties 
(P.I. Lacey, SAE Technical Paper 2001-01-1904)

In VMPAUTO laboratory with the use of rheometer Anton Paar MCR 102 the measuring of HTHS viscosity was defined in more “gentle” conditions, than it is specified in standards: actually we can modulate shear rate 105 1/sec at +150 °С. However even with these datas we can get interesting results.

Pic 2. Here we see results of oil viscosity of fully synthetic oil Shell Helix ULTRA AV-L 5W-30 which was used in VW GOLF 1.6 2006 year. New oil had HTHS viscosity 3.62 mPa*sec. But after 8.000 km HTHS viscosity decreased on 0.16 mPa*sec (-4.4%), it means that the viscosity became “warning”. So, all negative consequences described above can appear soon.

In the beginning of 2013 year scientific department of VMPAUTO has started development of a new multifunction oil additive. The name of this oil additive is “P14”. In spring 2014 they have started full-scale tests on different cars.

As you can see on the pic.2 use of “P14” almost didn’t change oil HTHS viscosity of new engine oil (-1.4%). In the same time use of “P14” with oil after 8000 km allowed restoring of HTHS viscosity up to initial level and increase it a little bit (+3.0%), that allowed long working of engine oil.  


Pic 2.

HTHS oil viscosity at + 150 °Сand shear rate 105 1/sec

Every value is an average mean of 100 measurements.