The oil data sheet: what do viscosity at 100 °C, TBN, ash and Noack tell you?

Somebody on a forum posts two tables from the data sheets of two 5W-30 oils and hands down a verdict: the first has a viscosity index of 175 and the second 160, a base number of 11 against 8, so the first is better, end of story. Sound familiar? In fact, some of those numbers have already been fixed by the viscosity grade and the quality class, and the rest show not how good the oil is but which engine it was made for.

In short, an oil data sheet (the technical data sheet, or TDS) is a table of typical figures that the oil maker measures on samples of its own product. Behind every line there is a laboratory method, usually American (ASTM), European (CEC) or their ISO and DIN equivalents.

“Typical” does not mean guaranteed

Let us start with the small print under the table. The big makers write roughly the same thing there: the values are typical of normal production, they are not a specification, and the figures may vary slightly between plants and between batches. In other words, what the oil maker guarantees is not the numbers in the table but that the oil meets the classes and approvals printed on the can. The numbers are for reference only, like the calorie count on a packet of biscuits.

Two more things. First, data sheets age: sheets dated 2019 or 2020 are still in circulation, and one of them explicitly cites class A3/B4 in the 2010 edition of the ACEA sequences. Second, some of the “numbers” are not measurements at all. About half of the high-temperature high-shear viscosity (HTHS) values that we collected from data sheets for the oil directory are written as “≥3.5”, “≥2.9” or “≥2.6”. That is the class limit, copied from an ACEA or SAE document, not the result of measuring that particular oil. The same goes for ash: “1.0-1.6” or “≤0.8” on a data sheet only means “we comply with the class”.

What each line shows

The main lines of an engine oil data sheet look like this.

Viscosity at 40 and 100 °C, and the viscosity index

What the numbers of an SAE grade mean is explained in the article on how to decode the SAE designation, and the limits of every grade are in the viscosity table. That gives us the first conclusion: the viscosity at 100 °C on a data sheet adds almost nothing to the grade on the can. An SAE 30 oil must fall between 9.3 and 12.5 mm²/s, and 12.1 for one and 11.8 for the other only means that both are 30s.

The viscosity index is calculated from those two figures: the higher it is, the less the oil thins as it heats up. Oils on hydrocracked and synthetic base oils, and with polymer thickeners, usually have a high index. In the data sheets we collected it is mostly 160-175 for passenger car oils and 130-155 for heavy-duty ones. But it is not a good basis for comparing oils: why two 5W-40 oils behave differently in a warmed-up engine is explained in the article on oil viscosity at operating temperatures. Whether it is good or bad for your engine, only the carmaker knows.

HTHS: viscosity where it is hottest

HTHS is the viscosity at 150 °C and a very high shear rate, that is, under the conditions in a crankshaft bearing. What it means and where the ACEA class limits lie is explained in detail in the article on ACEA classes. Here I will say just one thing: a higher HTHS does not mean a better oil. For C3 and A3/B4 it must be at least 3.5 mPa·s, for C5 and C6 from 2.6 to 2.9. Two 5W-30 oils with an HTHS of 3.0 and 3.5 are oils for different engines, not a “worse” and a “better” one.

The cold: CCS, MRV and pour point

The number before the W is set by two cold tests. CCS shows whether the starter can turn the engine over: for 5W the viscosity at −30 °C must not exceed 6,600 mPa·s. MRV shows whether the oil pump can move the oil: for 5W it is no more than 60,000 mPa·s at −35 °C. Take two data sheets of heavy-duty 5W-30 oils: in one the CCS at −30 °C is 4,780 mPa·s, in the other 6,200. The first is a little lighter in the cold, but both are honest 5Ws. Look at the temperature next to the figure: CCS at −25 °C and at −30 °C cannot be compared with each other.

The pour point, on the other hand, is the most misleading line. It is the temperature at which the oil still flows in a cooled test tube, and the SAE J300 standard does not use it at all. Translated data sheets sometimes call it the “freezing point”, and the reader concludes that an oil listed at −45 °C will start an engine at −40 °C. It will not. The engine will usually stop turning over long before the oil stops flowing in a test tube. For gear oils, Brookfield viscosity (ASTM D2983) is given instead of CCS: under SAE J306 it must not exceed 150,000 mPa·s at −40 °C for 75W, at −26 °C for 80W and at −12 °C for 85W.

Base number, ash and Noack

The total base number (TBN) is the reserve of alkaline additives that neutralise the acids from combustion products. But here is the catch: it is measured by two methods. ASTM D2896 titrates with a stronger acid and gives a higher figure; ASTM D4739 uses a weaker one, and the same oil reads lower by it. So check which method the data sheet states.

On top of that, the base number goes hand in hand with ash: alkaline additives mostly contain calcium or magnesium, and after the oil burns they turn into sulphated ash. Why ash harms a particulate filter, and how much of it the A/B and C classes allow, is explained in the article on diesels with a particulate filter. No wonder ACEA demands a TBN of at least 10 from the full-ash A3/B4, and at least 6 from C3 and C4 (mid- and low-ash), both figures by ASTM D2896. Heavy-duty oils have their own minimums by the same method: E4 at least 12, E7 9, E8 and E11 7. In the data sheets we collected, the base number of passenger car oils is mostly 8-10 and of heavy-duty ones 10-12. A C3 oil with a base number of 7 is not “weaker” than an A3/B4 with 11 — it is for an engine that must not be given a full-ash oil.

Noack is the loss in mass of an oil held at 250 °C for an hour. The lower the figure, the fewer light fractions the oil loses in a hot engine. ACEA limits it to 13% for most passenger car classes and 11% for C4. The difference between 9 and 11% in two oils that are both inside the class limit is most likely invisible in an ordinary engine.

Flash point and density are production-control figures and tell the car owner very little: an oil with a density of 845 kg/m³ is no “lighter on the engine” than one with 860.

Why oils cannot be compared by a single figure

Now back to the forum verdict. It is wrong for at least four reasons.

1. The class has already fixed the figure. Viscosity at 100 °C is limited by the SAE grade, HTHS by the SAE and ACEA classes, ash, base number and Noack by the ACEA class. Within those limits, “more” does not mean “better”.

2. The methods and conditions differ. TBN by D2896 and by D4739, CCS at −25 and at −30 °C: different numbers even for the same oil.

3. “Typical” values are not guaranteed. A different batch, a different plant, an old data sheet, and a difference of a few units is worth nothing.

4. The figures are linked. A higher base number usually means more ash, and a higher HTHS means an oil that is not for an engine designed for C5.

What is genuinely useful on a data sheet

I will say it again: an oil is chosen by the carmaker’s requirements — approval, class, viscosity — and the data sheet does not change that order. First the service book, then a can with the right approval and class; how to make sure the approval is genuine is explained in the article on how to check an oil approval.

After that, though, the data sheet can be useful. It will confirm that the oil is full-ash or low-ash, that its HTHS matches your class and not the neighbouring one, and that the maker has stated the methods rather than just round numbers. And if, instead of figures, it is wall-to-wall “≥” and “complies”, that is information too.

So an oil data sheet is not a school report but a description of the engine the oil was made for. If that description matches what your service book demands, you can read the rest of the numbers with interest, but you should not choose by them.