Motul vs Castrol Engine Flush: Which Is Stronger? (by SDS)

Motul is the stronger deposit-dissolver of the two. Its base is a light C10-C13 aliphatic hydrocarbon - in plain terms, a solvent. Castrol works more gently: 75-90% of it is paraffinic mineral oil, plus a trace (≤0.3%) molybdenum additive. A badly sludged but otherwise healthy engine leans toward Motul. A high-mileage engine, or one whose condition you’re unsure about, leans toward Castrol. Everything below comes from section 3 of both products’ safety data sheets; the “stronger/gentler” calls are our own conclusions from the composition, not claims printed on the sheets.

Motul or Castrol - which engine flush is stronger?

Motul is the stronger deposit-dissolver; Castrol is the gentler of the two. And that isn’t a line lifted from a shop listing - it comes straight out of the composition each maker wrote into its own safety data sheet, in section 3, the one that lists what’s actually inside. The word “stronger” is already our reading of that composition, because no data sheet measures cleaning power. We make that caveat once here - and it applies to every “stronger/gentler” in the piece.

Plenty of drivers treat these two flushes as twins - two little bottles you pour in before an oil change, one or the other, no real difference. But read the composition and it gets interesting fast: the bases are completely different. Motul is built on a solvent, Castrol on an oil. That’s the one thing you can state flat-out, because both sheets are lying right there on the table.

Motul Engine Clean Auto - 2023 sheet, PL version - is 50-100% C10-C13 hydrocarbons, meaning a mix of n-, iso- and cyclo-aliphatics with an aromatic content below 2%. In everyday language: a light, aliphatic solvent. The second listed component is a hydrotreated light paraffinic distillate (CAS 64742-55-8) at 10-25%. A fairly pure solvent-based formula.

Castrol Engine Shampoo / Motor Flush - 2025 sheet, NL/BE version - goes the other way. Its backbone, at 75-90%, is a hydrotreated heavy paraffinic distillate (CAS 64742-54-7) - that is, plain mineral oil, and one the sheet doesn’t even classify as hazardous. On top of that sit three lighter distillate fractions, each capped at 3%, plus a small molybdenum additive. An oil base, not a solvent base.

Before we go further - one caveat about versions, because it matters. The Castrol sheet we’re reading here is the NL/BE version (Netherlands/Belgium). Manufacturers do tailor formulas to specific markets, so if the bottle in your hands ships with a different sheet - a Polish one, or a different date - check the CAS numbers and ranges on your own copy. The conclusions below are only as good as the document they rest on.

What’s really inside? Motul vs Castrol by the SDS

In short: Motul is mostly an aliphatic solvent, Castrol is mostly mineral oil with a small molybdenum additive. Here’s the line-up straight from section 3 of both sheets - no dressing up.

When you read those ranges, keep one thing in mind. Motul’s “50-100%” and Castrol’s “75-90%” are the bands the maker prints on the sheet - not the exact recipe figures. They don’t even have to add up to 100%, because SDS ranges simply overlap and get rounded up. Motul’s main component reaching 100% is just a wide band; the point is that the aliphatic solvent is the dominant component, and the light paraffinic distillate makes up the rest. You won’t back out an exact recipe from these bands - the sheet was never meant for that.

The second point is detergents, and here it gets interesting. Castrol’s shop listings are sometimes wrapped in the phrase “detergent-based.” Except the sheet we have lists paraffinic mineral oil as the base and no classified detergent anywhere in it. Granted, an SDS lists only hazardous components, so an unclassified cleaning additive could in theory hide in the “other” portion. But three-quarters to nine-tenths of the contents, in this sheet, is documented oil, not a cleaning agent. Worth knowing before you reach for a product “because it’s the detergent one.”

Molybdenum in the Castrol - why a friction modifier in a flush?

Castrol adds a molybdenum friction modifier (MoDTC ≤0.3%) that Motul’s sheet doesn’t carry at all. It’s the only functional additive either sheet lists - and, frankly, an odd one for a flush. A flush gets drained out with the old oil, after all, not left in the engine for the long haul.

It’s worth pausing here. A friction modifier usually earns its keep in oil that stays in the engine for thousands of miles - that’s where MoDTC genuinely cuts friction on the mating surfaces. In a flush drained after a few minutes, its role is far less obvious, and the sheet doesn’t explain it. Either the MoDTC protects surfaces during that brief window when the engine runs on thinned, flushed oil with reduced protection - or, just as likely, it’s a leftover of the oil base and additive package the formula was built on. Either way: it’s a protective additive, not a cleaning one. It brings nothing to deposit dissolving.

What that means for the cleaning itself is inference, not sheet content, because an SDS doesn’t describe the mechanism. At textbook level: the aliphatic solvent acts as a thinner, working into soft oil deposits and sludge, thinning them out and carrying them off with the old oil - rather than acting like a surfactant scrubbing parts the way dish soap does. Castrol’s oil tends to flush and suspend loose contaminants instead. Neither sheet measures how much either formula removes, or how fast.

The MoDTC itself (chemically: a long-chain molybdenum alkyl dithiocarbamate) appears in Castrol’s sheet as a friction modifier. And here’s the honest bit: the sheet pins those H-codes on the molybdenum component itself at ≤0.3%, not on the finished blend as a whole. That’s a property of the additive, not an automatic verdict on the whole bottle.

One thing needs saying plainly about both products: despite the word “shampoo” in the name, neither sheet lists a classic detergent-dispersant package. Neither one “scrubs” with a cleaning pack - at least not one that shows up in section 3 as a hazardous component.

The hard numbers: what else section 3 tells us

Since the sheets are the only hard source, let’s pull the rest out of them - beyond the raw percentages. Here’s what the classification shows.

One clarification on the “aromatics” row, so we don’t overstate it. Motul states its aromatic content outright - below 2% on the main component. Castrol’s sheet doesn’t list aromatics as a separate component - the heavy paraffinic distillate may contain trace aromatics, but the sheet doesn’t declare them. That’s a difference in what the sheet records, not proof of “zero aromatics” - and certainly not an advantage of one flush over the other.

Beyond that, note where each hazard profile comes from. Castrol’s oil base is unclassified, and the whole “edge” of its hazard profile comes from the small molybdenum additive and some of the lighter distillate fractions. Two completely different stories.

Which flush is safer for an older engine?

For a high-mileage, tired engine, Castrol is the gentler pick. An oil base carries less risk of dissolving hard deposits than Motul’s solvent does - and in an old engine, those deposits sometimes double as a makeshift gasket.

The logic is simple, and every mechanic knows it from experience. The harder something dissolves, the better the chance it shifts not just sludge but the hard deposits sealing tired seals and dried-out rubber. The solvent-based Motul has more potential here - both to clean and to expose a leak that a deposit had politely been plugging. That’s not a fault in the product, it’s the nature of a solvent. The oil-based Castrol is calmer: for a high-mileage engine whose service history you barely know, an oil flush is the safer first move, with less chance of upsetting the fragile balance the thing is somehow still running on.

Don’t know the state of your engine? Start with the gentler one - Castrol. That’s the most common situation, and the rule then is simple. If you don’t know the service history or the real mileage, and can’t say whether there’s hard carbon inside acting as a gasket, an oil flush risks the least. You reach for the stronger, solvent-based Motul deliberately - when you know the engine is healthy and you want to shift a specific, stubborn deposit. “Gentle first, stronger only when I know I can” is simply safer than the reverse.

The hazard profiles differ for you, too, not just for the engine. Nitrile gloves are worth wearing with both, for different reasons: with Motul because of the solvent and skin drying, with Castrol because of the molybdenum’s sensitizing potential.

How do they differ from other flushes and from brands like TEC 2000 / Liqui Moly?

Both sit in the same category - products you add to the oil before a change. They differ mainly in the base: solvent (Motul) or oil plus an additive (Castrol). That’s what the sheets show; everything else depends on the specific formula.

The flush market is fairly crowded - pure solvent products, oily “conditioners,” some with anti-friction additives. Motul and Castrol are two opposite poles of the same shelf. If you care about what will most powerfully shift a hard deposit, Motul’s composition points the way. If you want the gentlest possible flush, you’re looking toward Castrol. The same sheet-by-sheet comparison for Motul and Mannol is in Motul vs Mannol engine flush.

And brands like Liqui Moly or TEC 2000? Those are separate formulas with their own safety data sheets. There’s only one honest way to compare them: take the sheet, read the composition, and see whether the base is a solvent, an oil, or a detergent package - exactly the way we did here with Motul and Castrol. If you’re building your own ranking and hunting for the best engine flush, do it by the composition on the safety data sheet, not the slogan on the packaging. You’ll find the TEC 2000 sheet in that same SDS line-up and can set it side by side, component for component.

What the SDS doesn’t tell you at all

This is the distinction most online comparisons skip - and it’s a shame. A safety data sheet is a safety document, not a user manual. It tells you what’s inside and what the hazards are. It does not tell you how to use the product, how much to use, or what it costs.

That’s why you won’t find, in this piece, numbers the sheets don’t contain - no recommended dose, no idle-running time, no price, no “% of sludge removed.” The maker defines that data on the label of the specific bottle, and that’s where you go looking. And if you see a hard claim online like “removes 85%,” treat it as a marketing line, not the result of an independent test. Neither of these two sheets backs up any such number.

Motul vs Castrol - who’s which for?

The decision shortcut below gathers what the composition implies. If you know the state of your engine, the choice is simple; for everyone else, the previous rule holds - when in doubt, start with the gentler Castrol.

Petrol or diesel? The sheets don’t split these products by fuel type - they describe the chemical composition, not a stated purpose for compression- or spark-ignition engines. Any restrictions - compatibility with a particulate filter or catalytic converter, say - are decided by the bottle’s label, not the SDS. If the packaging clears your engine type, go by that. And if you have doubts about the age and condition of the unit, the composition argues for the gentler option.

Frequently asked questions

Can you mix Motul with Castrol in a single flush?

There’s no reason to, and I wouldn’t. These are two different bases - a solvent and an oil - with different behaviors and different hazards. The sheets don’t anticipate that use and give no basis for combining them. Use one flush per its own label, and leave it there.

Can a flush harm my engine?

The risk is mostly with old, neglected engines where hard deposits act as a makeshift gasket. A stronger solvent (Motul) has more potential to shift such a deposit and expose a leak. In a healthy engine, used briefly and per the label, a flush isn’t dangerous. If you don’t know the engine’s condition, start with the gentler option, and change the oil filter after every flush.

How do the two bases differ chemically?

Motul is built on C10-C13 aliphatic hydrocarbons (a solvent) topped up with a light paraffinic distillate. Castrol is built on a heavy paraffinic distillate - that is, mineral oil - with three light fractions and a trace molybdenum additive. That’s the single most important difference the sheets show.

Why is there a molybdenum additive in the Castrol flush?

MoDTC is a friction modifier - an additive that usually protects surfaces in oil that stays in the engine for the long haul. In a flush drained out with the old oil, its role is less obvious and the sheet doesn’t explain it. The most sensible readings are either protecting surfaces during that brief run on thinned oil, or a leftover of the oil base the formula was built on. One thing is certain: it’s a protective additive, not a cleaning one - it brings nothing to deposit dissolving.

Is Castrol really “detergent-based,” like the listings say?

The 2025 sheet (NL/BE version) shows an oil base - paraffinic mineral oil at 75-90% - and lists no classified detergent. The “detergent-based” line is therefore hard to square with what the composition shows. An SDS lists only hazardous components, so an unclassified additive could in theory be hiding somewhere, but the base itself, in this sheet, is documented oil. Treat the claim with caution and check the sheet for your own bottle.

What do the H-codes on these sheets mean?

These are hazards to the person using the product - which is why gloves and ventilation are worth it in both cases.

How can I tell the flush worked?

Honestly: it’s hard to gauge by eye, and neither of these sheets claims any effectiveness - an SDS isn’t about effectiveness at all. An indirect sign is often a darker, more sludge-laden oil drained after the job. Don’t take the percentage claims from shop listings as test results - that’s marketing, and none of the sheets discussed here backs it up.

Last updated: July 2026.

Sources: Safety data sheet (SDS) Motul Engine Clean Auto, section 3, 2023 (PL); Safety data sheet (SDS) Castrol Engine Shampoo / Motor Flush, section 3, 2025 (NL/BE).