Viscosity (10ml)– This is defined as resistance to flow, or how thick an oil is.It changes with temperature i.e. as you heat an oil it will begin to thin and as the oil is cooled it will begin to thicken. The importance of viscosity is that at high temperatures the lubricant film must be thick enough to separate the two surfaces, but thin enough to still be energy efficient. Many diesel engines are produced with a lower viscosity base oils with special Viscosity Index Improver additives so at low temperatures they are fuel efficient in cranking the engine, but at high operating temperatures still give sufficient film thickness to prevent wear. These oils are called multi-grades (e.g. 15W40, 5W30 etc). Viscosity is measured at two temperatures.

  • Viscosity at 40’C – All lubricating fluids (as standard) are measured at this temperature and is the standard temperature to measure fluids for industrial applications with ISO Viscosity grade ratings.
  • Viscosity at 100’C – This is performed on all multi-grade (denoted with a “W” in the grade, where W = Winter) and SAE grade fluids used in engines and automotive gear oils.

Common Causes

Potential Result

Potential advice


· Contamination with a high viscosity product e.g. grease

· Aging of an oil (oxidation etc).

· Excessive sooting.

· Incorrect lubricant used.

· Leaking head gasket / coolant leak/water ingress.

· Heavy fuel oil dilution

· Increased cost

· Engine overheating

· Harmful deposits or sludges

· Blocked filters

· Component seizure

· Confirm oil grade

· Check air to fuel ratio.

· Inspect internal seals

· Check unit for excessive heat

· Check for leak injectors and fuel lines.

· Change oil (small volume systems) or top-up / renew with new oil on larger volume systems to bring back to correct oil viscosity.

o Send a sample of used and new oil to your Lubetrend Lab laboratory to help find volume required to achieve this.

· Change oil filter

· Inspect temperatures to localise hotspots (in and out of engine and cooler)

· Confirm oil level and top-up changes.


· Contamination with a low viscosity product e.g. diesel.

· Normal mechanical working of an fluid in multi-grade products (Polymer shear).

· Incorrect lubricant used.

· Increased cost

· Poor lubrication

· Metal to metal contact.

viscosity grade chart Viscosity
Download our viscosity classification handout here: LearnOilAnalysis Viscosity Grade Chart.

Find out how viscosity is tested below, what is it’s significance and how to diagnose viscosity changes. Then test what you learn in our test your knowledge quiz below.

Test your knowledge of viscosity.

Viscosity Quiz

Basic oil analysis test for beginners