Many drivers focus on tyres, brakes, and lights when preparing for an MOT, but engine oil quality is often overlooked. In reality, the condition and type of engine oil used can have a direct impact on MOT emissions and overall engine cleanliness. Understanding this connection can help UK motorists reduce the risk of an MOT failure and keep their vehicle running efficiently.
Engine oil is essential for lubricating moving parts, reducing friction, and preventing excessive wear. It also plays a key role in keeping the engine clean by suspending dirt, soot, and combustion by-products until they are removed during an oil change.
When oil quality deteriorates or the wrong grade is used, contaminants build up more quickly. This can affect combustion efficiency, which in turn influences exhaust emissions tested during an MOT.
Old or degraded engine oil becomes thicker and less effective at lubrication. This can lead to incomplete combustion, producing higher levels of carbon monoxide (CO), hydrocarbons (HC), and particulate matter. During the MOT emissions test, these elevated levels may exceed legal limits, particularly for diesel vehicles.
Sludge build-up caused by poor oil maintenance can also restrict engine components such as piston rings and valves. This reduces engine efficiency and contributes to dirtier exhaust gases, increasing the risk of a failed emissions test.
While the MOT does not directly inspect engine oil, cleanliness-related issues often reveal themselves indirectly. Excessive smoke from the exhaust, oil leaks, or contaminated exhaust systems may all be linked to poor oil condition.
For diesel cars, excessive smoke is a common reason for MOT failure. Dirty oil increases soot production, which can overwhelm the diesel particulate filter (DPF), leading to visible smoke and higher emission readings.
Manufacturers specify particular oil grades and standards for a reason. Using oil that is too thick, too thin, or not designed for your engine type can affect combustion temperatures and emissions control systems.
Modern vehicles, especially those with turbochargers or DPFs, rely on low-ash oils to reduce harmful deposits. Using the wrong oil may not cause immediate problems but can gradually worsen emissions performance over time.
Changing your engine oil and filter before an MOT can be a simple yet effective step, especially if the oil is old or contaminated. Fresh oil improves lubrication, reduces internal friction, and helps the engine burn fuel more cleanly.
Regularly checking your MOT status and emissions history through services like CheckMot can also help identify patterns that may point to maintenance issues, including oil-related problems.
Yes. Poor-quality or old oil can increase harmful exhaust emissions, raising the risk of an MOT failure.
If your oil is near the end of its service life, changing it before an MOT can help improve emissions performance.
Indirectly, yes. Incorrect oil can lead to higher emissions, smoke, or engine issues that may fail MOT checks.
Yes. Dirty oil increases soot production, which can worsen smoke levels during diesel MOT tests.
Most vehicles require an oil change every 6,000–12,000 miles, but always follow manufacturer guidance.