Fiat 9.55535-S1, S2, S3, DS1: what do the Fiat codes on the can mean?

The service book of a diesel Fiat says “9.55535-S1”, and the can on the shelf says “Fiat 9.55535-S2”. The salesman sees no problem: “It’s the same Fiat approval, and the 2 makes it newer.” It sounds convincing, and plenty of owners of Fiats, Alfas and Lancias buy their oil exactly that way. But S1 and S2 are not two revisions of one approval; they are two different oils for different engines.

What a manufacturer approval is, and why it matters more than the quality class, is explained in the article on what oil approvals are; the main point from there still holds: an approval is not a rung on a quality ladder but a narrow specialisation of the oil. And the article on Opel after GM has already mentioned that Stellantis is moving the lettered Fiat codes into a new FPW 9.55535 numbering. Here we look at the codes themselves, the ones you most often meet on cans.

How to read a 9.55535 code

9.55535 is the number of the Fiat standard for engine oils, and the letters and digit after the hyphen are a separate specification with its own viscosity, its own requirements and its own range of engines. On its own, “9.55535” says almost nothing, like the name of a town on an envelope with no street or house number.

For decades Fiat added new suffixes for new engines, and the old codes never went away: the cars they were written for are still on the road. So three or four Fiat codes in a row on one can is perfectly normal, as long as yours is among them.

In the newer codes the letters partly hint at the purpose: oil makers describe GS1 as a code for petrol engines, and DS1 and DSX for diesels. But there is no general “cipher”, and guessing from the letters is a bad idea: oil makers link S1, for example, chiefly with diesels, and S2 with petrol engines.

The most common codes: what stands behind each

The Fiat standards themselves are not publicly available, so what follows is what the oil makers themselves write about each code in their technical data sheets. Engines are named only where the sheets name them; unless stated otherwise, we mean Fiat, Alfa Romeo and Lancia engines.

There are rarer codes too: DH1 (a 0W-30 oil for commercial-vehicle diesels) and DM1 (a 0W-20 oil that the data sheets list next to PSA B71 2010).

Owners of a Jeep, Dodge or Chrysler should know that Chrysler gives the same requirements its own “mirror” numbers. MS-12991 corresponds to the Fiat 5W-40s (S2, M2, N2 and others), and the other pairs are these: GS1 and MS-13340, DS1 and MS-90047, GSX and MS-12145, S3 and MS-11106, G1 and MS-50081. What else to look for in the manual of a car brought in from across the Atlantic is covered in the article on oil for cars from the USA.

Why S1, S2 and S3 are not interchangeable

Look at the table again: three “neighbouring” codes differ in almost everything that can possibly differ.

1. Viscosity. S1 and, as a rule, S3 are 5W-30; S2 is 5W-40. And viscosity is part of the approval: the carmaker wrote it down for a particular engine, and you must not stray from it.

2. High-temperature viscosity, HTHS (in plain language, the thickness of the oil film in the most heavily loaded parts of a hot engine). ACEA C2, which underlies S1, allows an HTHS from 2.9 mPa·s, while C3, which underlies S2 and S3, requires at least 3.5 mPa·s. An engine designed for 3.5 mPa·s will get a thinner film from a thinner oil than its designers intended. And what may go into an engine built for a thinner oil is decided only by its maker.

3. Ash. S1, S2 and S3 belong to the mid-SAPS oils (sulphated ash up to 0.8%), S4 to the low-SAPS ones (up to 0.5%), while M2, N2 and Z2, with their A3/B4, are full-SAPS: they may contain up to 1.6% ash. Pour the “good old” M2 into a diesel with a particulate filter and the ash, which does not burn off when the filter regenerates, will gradually clog it. There is more on these classes in the article on the current ACEA classes.

So a bigger digit in the suffix does not mean “newer and better”: S2 is not the successor to S1 but a neighbour with a different viscosity, for entirely different engines. The same goes for the letters: GS1 and DS1 have the same viscosity and the same ACEA class, but they were written for different engines.

From the old codes to FPW 9.55535

After PSA and FCA merged into Stellantis, the group began to fold the standards of its brands into a single system: FPW 9.55535 with a number after the slash. Here the number is tied to the viscosity: 0W-20 (/01), 0W-30 (/02), 5W-30 (/03), 10W-40 (/04) and 5W-40 (/09 and /10); other numbers turn up too, for example /05. Numbers /09 and /10 are different approvals of the same viscosity: oils under /09 are ACEA C3, under /10 A3/B4.

Stellantis publishes no general “conversion table”, but in the data sheets of oils approved under FPW the makers list the old codes that such an oil can replace (“backward compatible”, as the data sheets put it). It comes out roughly like this:

• FPW 9.55535/01 (0W-20): among other things in place of GSX, DSX, DM1 and CR1;
• FPW 9.55535/02 (0W-30): in place of S1, GS1, DS1, DH1 and G1;
• FPW 9.55535/09 (5W-40, C3): in place of S2, T2, GH2 and M2;
• FPW 9.55535/10 (5W-40, A3/B4): in place of M2, N2, Z2 and G2.

In one such list S3 sits under /10, next to a full-SAPS A3/B4 oil, which hardly goes with the C3 underlying S3; S4 is not there at all. For these, consult the documentation for your car. And mind the direction: an oil with a new code goes where the old one was required, not the other way round. A can labelled “S1” does not become a /02 oil just because the data sheet of some /02 oil has a line about S1.

Why do the cans at Fiat dealers say Selenia? The brand grew out of Fiat’s lubricants division and, even after becoming a separate company (it now belongs to Malaysia’s Petronas), it kept supplying the factory fill for Fiat. The data sheets of newer Selenia oils say outright that they were developed together with FCA, and Stellantis calls the partnership historic. According to a 2025 trade press report, TotalEnergies won the factory fill and aftermarket supply for Stellantis for five years from 2026. All you need to take from this is that the brand on the can at the dealer may change, but the code in the service book will not. Neither brand is somehow “more correct”; the code decides.

And one more source of confusion. 9.55550 is not about engine oils at all: that is the number under which Fiat writes its requirements for transmission fluids. MZ6, for example, is on 75W-80 GL-4 oils for manual gearboxes, AV1 or AG1 on fluids for automatics, HE2 on fluids for dual-clutch gearboxes. They are chosen the same way, by the code in the documentation.

How to check an oil against a Fiat code

1. The full code from the service book, suffix included. “9.55535” without letters is only half an address.

2. The label and the data sheet. A genuine approval is marked with the words “approved” or “approval”, and on the data sheets of some oils there is also a CTR approval number beside it. The same code after “meets”, “performance”, “suitable for” or “complies with” is the oil maker’s own claim. And that is what most of them are: among the oils in our catalogue the Fiat codes almost always appear as “meets the requirements”. How to tell these wordings apart is in the article on how to check an oil approval.

3. For FPW, the revision too. These standards get rewritten: /03, for example, has several revisions. After the British verification body VLS looked into claims of “meets /03”, several sellers removed the wording or specified on the label which revision the oil meets.

4. No book? VIN and the dealer. Fiats of several generations, and especially cars with engines from other groups (Renault diesels under S4, American engines under CR1), are exactly the case where guessing from engine size will not do. And if your documentation says PSA B71, you need the article on Peugeot and Citroën approvals.

I will say it again: a Fiat code can only be read in full — number, hyphen, suffix — and swapping one suffix for another “because the digit is bigger” is not allowed. And if the seller insists that “S2 is the same as S1, only newer”, ask to see it in the service book of your Fiat.