by If you have a turbo engine with over 150,000 km (93,000 miles) and the lubricant is gradually decreasing, a natural question arises: should you switch from 5W-30 to 5W-40? It often seems that simply buying a thicker engine oil will solve the problem. In practice, everything depends on the cause of the oil consumption and the manufacturer’s approval, not just on the thickness.
Engine Oil: Why Does Consumption Appear After 150,000 km?
In a turbocharged engine, wear manifests itself faster due to high temperature and load. Over time, thermal clearances increase, seals lose elasticity, and turbocharger gaskets age. As a result, car oil can be lost through the intake or exhaust.
If you notice oil consumption, it’s important to first understand its source. If the cause is mechanical wear of the turbocharger or coked-up piston rings, changing the viscosity will not fix the problem. In this case, even the highest quality oil in the engine will only partially reduce the symptoms.
5W-30 or 5W-40 Motor Oil: When is the Switch Justified?
The main rule is to refer to your vehicle’s manual. If the manufacturer allows both viscosities, a switch is possible. If the manual specifies only one, you should not change the viscosity grade.
What you can check when buying motor oil:
- The SAE viscosity grade is indicated on the front of the canister (e.g., 5W-30 or 5W-40);
- Automotive manufacturer approvals (VW, BMW, MB, Renault, etc.) are also indicated on the packaging or the store’s website;
- The API and ACEA categories are placed on the back of the canister.
The HTHS (High-Temperature High-Shear viscosity) indicator is most often not indicated on the canister itself. It can be found in the technical data sheet for motor oils on the manufacturer’s website. If you are considering switching to a more viscous engine oil, check that the HTHS meets the requirements of your approval.
A study by SAE (Society of Automotive Engineers, 2020) showed that in turbocharged gasoline engines with over 120,000 km (75,000 miles), using a lubricant with an HTHS of 3.5 mPa·s reduced volatility by 12-15% compared to an HTHS of about 3.0 mPa·s. This means that with wear, higher thermal stability can indeed reduce oil burning if the engine is technically sound.
Car Oil: When Should You Not Change Viscosity?
Before changing the viscosity grade, it is important to understand whether a thicker oil in the car will create additional risks. Below are systematized situations in which switching to a higher SAE grade is not technically justified.
| Situation | What This Means for the Engine | Why You Shouldn’t Change Viscosity |
| The manufacturer allows only one SAE grade | The design of components is calculated for a specific lubricating film thickness. | The calculated hydrodynamics of the system and thermal balance are disrupted. |
| Oil consumption is related to a faulty turbocharger | Turbocharger seals or bearings are worn. | A thicker lubricant does not eliminate the defect or restore seal integrity. |
| Unstable pressure in the system | Possible severe pump wear or increased bearing clearances. | A thick formula only masks the symptoms without compensating for mechanical wear. |
| Operation in a cold climate | Frequent cold starts at consistently low temperatures. | Increased viscosity slows down pumping and the delivery of lubrication to friction components. |
A more viscous lubricant indeed forms a denser film at high temperatures, but during a cold start, it moves through the system more slowly. In a turbocharged engine, this affects the lubrication speed of the turbocharger bearing, which is particularly sensitive to delays in oil delivery to the engine components.
Automotive Oil: Sometimes the Cause of Consumption is Not Viscosity, but the Operating Mode
In a turbocharged engine, oil consumption can be related not only to wear but also to the operating mode. The turbocharger operates at very high temperatures, and if the engine is shut off immediately after active driving, the car oil remains in the overheating zone. Over time, this leads to partial coking in the hot turbocharger housing, impairs heat dissipation, and increases oil burning. In this case, switching to a thicker lubricant does not eliminate the root cause.
Before buying engine oil of a different viscosity, check:
- Whether you are exceeding the recommended oil change interval.
- Whether you let the engine idle for 1-2 minutes after active load.
- Whether the chosen lubricant matches the operating conditions — city, highway, climate.
Sometimes adjusting the maintenance schedule and shortening the oil change interval reduces consumption more effectively than changing the SAE grade. In a turbo engine, the stability of characteristics and the manufacturer’s approval are more important than the thickness itself.
