Oil Analysis 101


Top Post Dawg
Dec 2, 1999
Audi 100S

A: By taking a small amount (3-4 oz) of used oil and having it tested, you can determine the types and relative amounts of wear metals and contaminents (soot,water,fuel,sludge) in the oil. There are also specific tests to determine how the oil is holding up physically/chemically and how much of the additive package (anti-wear and detergent/dispersant additives) remains.


1) Is the component (engine/transmission/differential/power steering system, etc) functioning "normally"?

2) Is the lubricant good for continued use and if so, about how much longer is it good for?

Oil analysis isn't a new technique - it's been used by the aviation industry for decades to monitor the condition of jet turbine engines. It's also widely used to evaluate industrial compressors and by trucking and construction companies to monitor HD diesel engines,hydraulic systems,
etc. A complete tests typically runs $15.00-$20.00 and results are available in about 2 weeks ....


Wear metals: Usually expressed in terms of Parts per million (ppm). Allowable limits are generally on the order of 100 ppm for iron and 50 ppm for lead/copper/aluminum. Most other wear metals are going to be detected at tract amounts. Some metals, such as Moly, are actually oil additives. Zinc and Phosphorus are the two most widely used anti-wear additives. Calcium and Magnesium are detergent/dispersant additives - high levels of these are DESIRABLE in oil analysis reports.

CONTAMINENTS: (Allowable limits)

Soot: at or below 3.0% (my recommendation for the TDI, based on engine design). High levels will hinder proper oil circulation and increase abrasive wear.

Water: below 0.1% (may indicate coolant leak into lubricating oil)

Fuel: below 0.5% (high levels may mean leaking or clogged injectors)

Ethlyene Glycol: SHOULD BE NEGATIVE - even small (0.5%) amounts can be very bad. Change oil immediately upon detection and pressure test cooling system.

Total solids (soot,dirt,sludge, wear metal content): below 2.0%

OIL VISCOSITY (increases due to the buildup of wear metals and contaminents; decreases due to polymer shearing and fuel dilution)

Should stay in grade ...

For 30wt oils: 9.3-12.5 Centistokes @ 100C
For 40wt oils: 12.5-16.5 " " "


Oxidation: Due to the reaction of oxygen attaching itself to the hydrocarbon chains to form an insoluble polymer (sludge). Test indicates a percentage (%) of the allowable limit.

Nitration: Oxides of Nitrogen (NOx) combines with moisture to continually form minute amounts of Nitric acid - this acid is neutralized by the alkaline detergent additives (remember titration from HS chemistry?). Test indicates a percentage (%) of the allowable limit.

Total Base Number (TBN): A measure of reserve alkalinity - ie, how much more acid can the oil neutralize? TBN's for HD diesel oils are generally in the 9.0-12.0 range ...when the TBN drops below 50% of the original value the oil should be changed. Retention of TBN is more important than simply having a high initial number. In diesels sulphuric acid is also formed due to the sulphur in the fuel (a max of .05% with current EPA regulations).

Finally, it is important to have the oil FULLY MIXED UP when taking a sample, so that it is representative of the bulk oil properties. The best method is a petcock valve that allows sample taking with the engine warmed up and running. Next best is extracting a sample through the dipstick tube within 10 minutes after shutdown - the sample should be drawn from midway down in the sump(measure the tube against the length of the dipstick) - not off the bottom of the oil pan. You can also "catch" a sample when changing oil from the drain plug (it's messy at best). Do not let the oil settle before taking a sample - the lead and iron will tend to settle out and you'll get results that aren't representative of the oil condition.


[ July 22, 2001: Message edited by: TooSlick ]


Man this is why I love this site, some real good information I can use over and over again and is worth saving and printing out! Thanks TwoSlick!

As everyone is aware, Ric is having his oil professoinally analysed by a team of African Safari Big Game Hunters (Buwanna & Co.) who have a sideline business in snake oil analysis and weight loss dieting. When he gets his data back from these trophy head hunters, I am certain we will all be entertained with the long life his TDI is blessed with