Hydrocracked oils
In this section: 7 articles
What are hydrocracked engine oils, how do they differ from mineral and synthetic ones, and does a car owner even need to know any of this? Sometimes I get the impression that the scary-sounding word "hydrocracking" was cooked up by some brilliant marketer purely to carve up the market.
Because if you dig a little deeper into the production technology behind those same automotive oils, you can unearth even more "scary" and hard-to-grasp terms.
So let's try to figure out whether it really matters to a car owner — hydrocracked oil or something else...
I'll say it straight away, so as not to keep you in suspense: for most drivers the answer is "hardly at all". But the word "hardly" is the key one here, so let's go in order: what oils are made from, what exactly hydrocracking does, why the writing on the can means so little, and when a more expensive base oil really earns its money.
The five groups of base oils in plain words
Any engine oil is a base oil, which makes up most of the volume, plus an additive package. The American Petroleum Institute (API) divides base oils into five groups. It divides them by measurable properties (sulphur content, saturates content, viscosity index), not by advertising words:
| Group | How it is made | What the can usually says |
|---|---|---|
| I | petroleum base oil refined with solvents | mineral |
| II | petroleum base oil refined with hydrogen (hydrotreating, hydrocracking) | mineral |
| III | deep hydrocracking; GTL base oils belong here too | synthetic, “synthetic technology”, HC-Synthese |
| IV | PAO (polyalphaolefins), synthesised at a chemical plant | synthetic |
| V | esters and other base oils that did not fit into the first four groups | synthetic; often an addition to other base oils |
Semi-synthetic is not a separate group: it is a blend, most often of a mineral base oil with a Group III or IV one. And GTL (gas-to-liquids) is an interesting special case: this base oil is made not from crude oil but from natural gas. It comes out very clean and with a high viscosity index, but formally it also belongs to Group III; in the industry such base oils are unofficially called III+. We went through the limits of each group, with figures, in the article “Synthetic” on the label; the general picture is enough here.
What hydrocracking actually does
An ordinary mineral base oil is a petroleum fraction from which solvents have washed out the worst. The sulphur, aromatic hydrocarbons and unstable molecules stay in it, although in smaller amounts. In hydrocracking the feedstock is treated with hydrogen at high pressure and temperature over a catalyst. The sulphur and nitrogen go away, the aromatics are saturated with hydrogen, and long branched molecules are broken up and rebuilt into more uniform ones.
What does this give the engine? A high viscosity index (the oil thins out less in a hot engine and thickens less in the cold), better resistance to oxidation and lower volatility than mineral oil. That, in fact, is why a Group III base oil was able to become "synthetic": on most of the parameters that matter to an ordinary engine it is closer to PAO than to mineral oil. There are weak spots too: in severe frost and at very high temperatures PAO and esters, as a rule, still behave better. More about the technology itself in the article what is hydrocracking.
Why the word "synthetic" guarantees little
Legally, this word is hardly protected anywhere. In the USA in 1999 the National Advertising Division (NAD) heard Mobil's complaint against Castrol and agreed that an oil based on Group III could be advertised as synthetic. Since then most manufacturers have done exactly that. Germany took a different path: in 2018 the Federal Court of Justice (BGH) found the words "fully synthetic" on an oil with a hydrocracked base misleading, which is why on German cans you can see the more honest "HC-Synthese". The EU as a whole has no legal definition of "synthetic" for engine oil, so "synthetic" on a can may mean PAO, esters or ordinary Group III alike. You cannot tell them apart by eye or by touch either; more on that in the article how to tell hydrocracked oil apart.
What really matters to the driver
The engine does not care what the base oil is called. What matters to it is that the oil has the right viscosity, withstands the load, does not oxidise before its time and keeps the engine clean. And that is checked not by the name of the base oil but by tests of the finished oil. So there are three things to look at, in this order. First, the car manufacturer's approval (VW 504 00/507 00, MB 229.5, BMW Longlife-04 and so on), if your service book requires one. Second, the ACEA or API class and the SAE viscosity, also from the owner's manual. Third, and only when everything else matches, the price and the seller's reputation.
The word "synthetic" is missing from this list for a reason. If an oil on a Group III base has the required approval, it has passed the same engine tests as a PAO oil with the same approval. What approvals are and how to check them is in the article what are oil approvals.
Hence the advice on how to read a label. Read the can from the back, not from the front. The front is marketing: "synthetic", "advanced", "technology". The back is the substance: viscosity, the ACEA/API classes and the list of approvals. Look for the words "approved", "approval", "Freigabe" next to a car manufacturer's code. Wording such as "meets", "suitable for" or "meets the requirements" only means that the oil maker itself considers the oil suitable, without any official approval from the car manufacturer.
When it is worth paying extra for PAO or esters
There are situations where a more expensive base oil makes sense even with the same approval. The first is severe frost: if the car spends the night outdoors at minus thirty or lower, a 0W oil on PAO usually reaches the bearings and camshafts faster on a cold start. The second is heavy loads and high temperatures: boosted turbo engines, track days, towing a heavy trailer in the heat. PAO and esters evaporate less and oxidise more slowly, and these are exactly the properties that count here. The third is very long change intervals. If you really do drive to a flexible service interval of up to 30,000 kilometres, a reserve of base oil stability does no harm. But even here, look at the approval first: it is what decides whether the oil is allowed such an interval at all.
For everyone else (city driving, an ordinary climate, changes on schedule or more often), a hydrocracked oil with the required approval is, in my view, the smartest choice for the money. In such a regime, the extra paid for "real" synthetic mostly buys a reserve that the engine will never use. That money is better spent on a timely change and a good filter.
So hydrocracked oil in a can, mineral or "full" synthetic is, for the driver, a second-order question, after approvals, class and viscosity. If you want to dig deeper, read on through this section: why the base oil technology is secondary I explained in detail in the article are hydrocracked oils good or bad, and the summing-up of the whole topic is in the article can hydrocracked oils be used.