SLUDGE AND ENGINE OIL OXIDATION: CAUSES, RISKS AND EFFECTIVE SOLUTIONS (2025)

🧩 What is sludge and why does it form?

Sludge is a thick, sticky residue that forms when engine oil loses its detergent properties. It consists of carbon, lacquers, and impurities resulting from combustion and oxidation. The main causes are:

• Oil changes too far apart or dirty filters;
• Short journeys and urban use (oil always cold);
• Low quality oil or a mixture of different types;
• Traces of fuel or coolant in the lubricant;
• High temperatures and poor ventilation in the oil pan.

🧪 Oil Oxidation: What Happens Chemically

When oil comes into contact with oxygen and heat , its molecules oxidize and lose effectiveness. Detergent and antioxidant additives degrade, causing:

• Increased viscosity and reduced flowability;
• Formation of internal lacquers and sludge ;
• Reduction of lubricating power and accelerated wear ;
• Risk of clogging of the suction pipe and hydraulic tappets.

🚨 Symptoms and risks for the engine

• Oil cap encrusted or with brown residue;
• Very dark and thick oil after just a few thousand km;
• Mechanical noise when cold or unstable oil pressure;
• Engine warning light or abnormal oil level;
• Reduced performance and higher consumption.

🧼 Internal cleaning (flush) and safe procedure

To remove sludge and oxidation, it's essential to perform an internal cleaning (flush) before changing the oil. This helps unclog the passages and return the new oil to a clean environment.

LIQUI MOLY PRO-LINE – ENGINE WASH 500ml (SKU: 2427)

Professional cleaning additive for the engine oil system. Dissolves sludge and residue, improving lubrication and extending engine life before oil changes.

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LIQUI MOLY ENGINE FLUSH – OIL ADDITIVE 300ml (SKU: 2678)

Detergent additive for internal engine cleaning . Removes sludge and deposits before oil changes, improving lubrication and the life of mechanical components.

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Recommended frequency: every 2 services or at every oil change on engines with heavy urban use.

⚙️ After the change: recommended oils and additives

After flushing, it's essential to choose a high-quality oil to prevent further oxidation and sludge buildup. Choose fully synthetic formulations with high cleansing power.

🛢️ Valvoline oils: the right choice against sludge and oxidation

Valvoline oils are designed to keep the engine clean and the oil stable even under severe thermal conditions (turbocharged, tailpipes, urban use). Their strength lies in the balance between detergency/dispersibility and oxidation resistance , key factors in preventing sludge, lacquer, and carbon residue.

🔍 Because they reduce sludge and oxidation

• Advanced detergent/dispersant package: keeps impurities in suspension, preventing them from agglomerating into sludge.
• Highly stable antioxidants: slow down the degradation of the lubricant at high temperatures and in the presence of oxygen.
• Premium synthetic bases: more stable and cleaner lubricating film over time, with less tendency to form varnish in the ducts.
• Hot viscosity control: reduces oil thickening typical of prolonged oxidation.

🧭 Which Valvoline to choose (based on the scenario)

*Specifications are indicative: always respect the grade and OEM approvals indicated by the vehicle manufacturer.

VALVOLINE 908743 – ALL-CLIMATE C3 5W40

100% synthetic high-performance engine oil for diesel, turbocharged, and DPF-equipped engines. Offers excellent wear protection, internal cleanliness, and quick starts even at low temperatures.

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VALVOLINE 907855 – SYNPOWER MST C3 5W30

100% synthetic engine oil for Euro 4/5 vehicles with DPF, TWC and EGR . Ensures optimal protection, internal cleanliness and maximum performance in VW, Mercedes, BMW and Fiat engines.

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VALVOLINE 907818 – SYNPOWER XL-III C3 5W30

100% synthetic engine oil developed for VW, Audi, Seat and Skoda Euro 4/5 engines equipped with a diesel particulate filter (DPF) . Offers superior protection, long life and maximum efficiency even under extreme conditions.

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🧑🔧 Usage Guidelines (for concrete results)

• Interval: Change oil every 10–12,000 km (or 12 months) in urban use; change sooner if you notice thickening or a fuel odor.
• Periodic flush: Perform an internal cleaning every 2 services to keep the ducts clean.
• Don't mix: Avoid mixing different oils; if you need to change families (e.g., from 5W-30 to 5W-40), do so at the service.
• DPF: on diesel engines with a filter, prefer Low SAPS formulations (e.g. MST C3 5W-30) to protect the FAP and reduce deposits.
• Climate and driving style: high thermal stress (mountains, towing, summer motorways) ⇒ favour oils with high hot stability.

Note: Always check the technical data sheet and OEM approvals required by your vehicle manufacturer before purchasing.

🛡️ Preventive maintenance

• Change the oil every 10–12,000 km or once a year.
• Always use oil with the correct OEM and ACEA specifications .
• Do not mix oils from different brands.
• Flush every 2 services if you use your car in the city.
• Always warm up the engine before pushing it, especially when cold.

❓ Frequently Asked Questions (FAQ)

🏁 Conclusions

Sludge and oil oxidation are not inevitable: they can be prevented with periodic flushes , regular oil changes , and highly thermally stable lubricants. In the workshop, we see that a correct routine (flush → quality full synthetic oil → compliance with OEM specifications) significantly reduces deposits and keeps the engine efficient for longer.

• First: Perform a pre-cut flush to clear the ducts.
• Next: Choose a premium, high-detergency oil (e.g. Valvoline SynPower/MST/MaxLife depending on the scenario).
• Always: Respects 10–12,000 km (or 12 months) intervals and OEM approvals .

Not sure which oil to choose? Contact us: we'll help you find the right oil and additives for your vehicle's specifications and driving style.