How to read a used oil analysis report

A used oil analysis is one of the cheapest looks inside an engine you will ever get. Blackstone Laboratories' standard test costs $40 as I write this, and it tells you what metal the engine is shedding, what is getting into the oil that should not be, and whether the oil itself is still doing its job. The catch is that the report is a page of numbers, and the numbers only mean something once you know how the lab reads them. This is my plain guide, built on the guidance labs publish themselves.

Start where the lab starts: the comments

Blackstone's own advice is to read the comments first. A person at the lab has already compared your sample with averages for your engine and with your past reports, and written down whether wear looks normal, whether anything is high or unusual, and whether you need to act. Everything below helps you understand why they said what they said.

The standard report covers four things:

• A spectral exam for wear metals, additives, silicon and signs of coolant.
• Flashpoint, for fuel, solvent and moisture contamination.
• Viscosity, the thickness of the oil.
• Insolubles, the solids in the oil, which say how well the filter is keeping up.

TBN and TAN, which measure remaining additive and acidity, are optional extras at Blackstone. The elements are reported in parts per million (ppm).

Wear metals: what each one usually means

Small amounts of metal are normal; every engine wears. What matters is how much, compared with what. Blackstone's gas and diesel engine guide lists the usual sources:

Notice how many metals have more than one source. That is why labs look at the balance of metals, not just each one alone. In one example Blackstone gives, lead was not twice the average, but it was marked anyway, because lead and iron should run at roughly one to one by the averages and that sample was nearer four to one. More lead than iron says the bearings are wearing faster than the rest of the engine.

A real example: Blackstone has published a report from a 2008 BMW M3 with the S65 V8, where lead read 78 ppm against a universal average of 7, with iron, aluminum and copper also above average. Even with numbers like that, the lab said the engine may have a bearing problem and that it was hard to be sure from a first sample.

Contaminants: dirt, coolant, fuel and water

Silicon. Blackstone lists airborne dirt getting past the air filter as a source, but also sealers, gaskets, lubricants, antifreeze inhibitor and oil additives. High silicon after a gasket job means something different from high silicon with a torn intake boot.

Sodium and potassium. Both are listed as coming from antifreeze. Sodium can also come from oil additives, and potassium from aftermarket additives. When they rise together, coolant is the question to ask. Blackstone also reports an antifreeze percentage directly, and a question mark there means it found possible traces but not enough to say for certain. Its guide is blunt about why it matters: a serious coolant leak can destroy an engine.

Fuel. The lab reports fuel as a percentage, and too much fuel thins the oil and reduces lubrication.

Water. Blackstone's guidance is that none should be present.

Viscosity, flashpoint and insolubles

Viscosity checks whether the oil is still the grade it should be. Blackstone says it is not usually a problem when it is only slightly off. If it is much thinner than it should be, fuel or solvent contamination is the usual suspect. If it is thicker, excess heat, oxidation, soot or an aftermarket additive can be the cause.

Flashpoint is the lab's fuel detector. When fuel or solvent is present, the flashpoint reads low. A low flashpoint and a thin viscosity on the same report tell the same story twice.

Insolubles are solids, typically carbon from oxidised oil plus combustion blow-by past the rings. Blackstone says excess heat or a filter problem can push them up, and that whether a given level matters depends on how the engine is used.

Universal averages, and why the trend beats any single report

The universal averages column is Blackstone's baseline: typical wear for your type of engine, drawn from the samples it has tested. Your own unit averages sit beside it once you have history. A few things are worth knowing:

• Tell the lab your exact engine. Blackstone points out that one vehicle can come with several engines, each with its own averages, and that comparing against the wrong set hides subtle problems.
• Additives will not match the averages. Molybdenum, boron, calcium, magnesium, phosphorus and zinc are mostly oil additives, and the averages are a mix of every oil out there, so differences there are expected.
• Twice average is the rough flag. Blackstone generally marks a metal in bold when it is twice the average or more, but not always, because sometimes the reason is known.

Above all, Blackstone is clear that there is no magic number. It assesses each engine on how the sample compares with your own trend, with the average, and with the balance of metals, and it says it takes about three samples to get a good trend going. Its examples cut both ways. One engine ran more lead than average on every sample, held steady, and was simply watched. Another read just 12 ppm of lead, well within average, and the lab still flagged it, because that engine had always run much lower.

One more limit to keep in mind: the spectrometer cannot see everything. Noria's guide to iron in oil notes that the instrument commonly used for wear metals typically cannot detect particles larger than about 5 to 8 microns. A part that is shedding big chunks can read lower than you would expect, which is why some owners also cut open the filter.