Servicing by mileage or by time – which matters more

Many car owners who visit our service center ask the same question: what matters more when planning vehicle maintenance–mileage or time? For example, if the odometer shows only 5,000 kilometers and the car has spent most of its time parked in an underground garage or outdoors, it is tempting to postpone a service visit. After all, the engine oil still looks clean, the recommended 10,000–15,000 km service interval has not yet been reached, and the mandatory Polish roadworthiness inspection (badanie techniczne) has already been completed.

However, if you carefully read the manufacturer’s recommendations in the service manual, you will notice that they typically advise servicing the vehicle either after a specified mileage or every 12 months–whichever comes first. This is due to basic chemical and physical processes that continue to occur inside the vehicle, even when it is not being driven. Let’s take a closer look at why this matters.

Why Engine Oil Ages Even When the Car Isn’t Moving

In a closed–but not completely airtight–engine, the oil is constantly exposed to oxygen. This initiates oxidation, causing the protective additive package to gradually degrade while the oil base loses its viscosity characteristics.

Short trips make the situation even worse. If you start your car once a week just to drive to the nearest Biedronka or pick up your child from school, neither the engine nor the exhaust system has enough time to reach normal operating temperature (typically 90–100°C). As a result of temperature fluctuations, condensation inevitably forms inside the crankcase. The water from this condensation mixes with small amounts of unburned fuel, soot, and blow-by gases, creating an aggressive acidic emulsion.

When a vehicle is driven regularly over longer distances, the moisture evaporates from the hot engine oil. But if the car is used only occasionally for short trips, the acidic environment literally consumes the active additives over the course of a year. By the end of that period, the oil may have lost most of its protective properties–even if the car has covered only 3,000 km.

What Happens to Other Consumables During a Year of Minimal Use?

The engine is not the only component affected by time. Other vehicle systems also continue to age, even when the car spends most of its life parked.

• Brake fluid is hygroscopic, meaning it absorbs moisture from the air through microscopic pores in rubber brake hoses. Within two years, the water content can reach a critical 3–4%. As a result, the fluid’s boiling point may drop from a safe 230°C to around 150°C. During a long downhill drive or emergency braking, this can cause the brake pedal to become soft or even lose effectiveness. Excess moisture also accelerates corrosion of brake pistons and wheel cylinders.
• Rubber components and drive belts gradually dry out and lose elasticity due to aging, temperature fluctuations, and the damp conditions typical of Warsaw winters. A timing belt that cracks because of age is just as dangerous as one worn out by high mileage.

Why Should the Recommended Service Interval Be Cut in Half for City Driving?

If you drive your car every day but almost exclusively in urban traffic, the standard 15,000 km oil change interval may already be too long. The key factor here is engine operating hours–the actual amount of time the engine spends running.

Let’s compare two scenarios:

• Highway driving. On roads such as the S8 or A2, the average speed is around 90 km/h. Covering 15,000 km takes approximately 166 engine hours, with the engine operating under optimal temperature and load conditions.
• Warsaw during rush hour. Traffic jams on the Grota-Roweckiego Bridge, crawling along Aleje Jerozolimskie, frequent traffic lights, and extended idling reduce the average speed to around 20 km/h. The same 15,000 km now require approximately 750 engine hours.

Most hydrocracked engine oils reach the end of their effective service life after roughly 250–300 engine hours. This means that by waiting until the odometer reaches 15,000 km while driving mainly in Warsaw traffic, the engine may spend an additional 400 hours running on oil that has already oxidized and lost much of its protective capability.

RemZone Checklist: How to Plan Vehicle Maintenance Properly

To protect your engine from premature wear and avoid expensive repairs, we recommend following these simple guidelines:

• If your annual mileage is below 5,000 km, replace the engine oil and oil filter every 10–12 months. Do not wait until the mileage limit is reached.
• If your vehicle is driven mainly in city traffic and frequent traffic jams, shorten the oil change interval to 7,500–8,000 km or change the oil based on engine hours–no later than after 250–300 hours of operation.
• Replace the brake fluid every two years, regardless of mileage. During your annual badanie techniczne inspection, ask the technician to measure the moisture content of the brake fluid using a dedicated tester.
• Replace the coolant every 3–5 years, depending on its type (G11/G12/G12++), as its anti-corrosion additives gradually deteriorate and lose their effectiveness over time.

Performing scheduled maintenance according to the calendar is not a marketing gimmick–it is the only reliable way to keep the engine clean internally and ensure that your vehicle starts reliably and brakes safely whenever you need it.