Why ‘Max’ is not always right

Posted on July 29th, 2013 by Rob Marshall

Why ‘Max’ is not always rightAt least several times a year, various oil companies remind drivers to check their vehicles’ oil levels more often. While a cynical onlooker might conclude that their aim is to sell overpriced one-litre top-up bottles, the advice is still sound. The most recent survey, conducted on behalf of Mobil One, revealed that not only are engine oil levels not been checked weekly but, in addition and even more worrying, 20% of car owners ignore the glow of a fascia light warning.

Yet, many companies recommend that more oil is needed, should the level fall below the dipstick’s maximum mark. Naturally, this presumes that the car has a manual dipstick and not all of the latest models possess one. Although the ‘maximum’ is the optimum level for most engines, many modern common-rail diesel units are failing, due to being overfilled. This somewhat ironic situation is caused by either too much oil being added in first place, or the level increasing, due to diesel fuel contamination, during the active regeneration cycles of the diesel particulate filter (DPF).

I interviewed a senior technician of a well-known car company recently, who confirmed that, for some modern diesel engines, filling the oil levels to the maximum mark reduces any safety margin, should diesel contamination occur. The consequences of driving with an elevated sump level can be just as serious as running an engine with insufficient oil. Excessive lubricant can be pressurised and can pass by internal seals, causing a serious leak. Perhaps worse, oil can be forced into the intake system and combusted, which can cause uncontrolled engine runaway. He advised that, for some diesel engines, the optimum oil level should be around the half-way mark, to provide an allowance for potential diesel contamination. He emphasised the importance not to exceed the recommended oil change intervals and also to use the correct low SAPS oil for any vehicle equipped with a DPF. Advice on choosing engine oil can be found here.

Therefore, on modern diesel cars, fitted with a DPF, topping the oil level up to its maximum mark might not be the commonsense advice it once was. Yet, always consult a vehicle technician first, one who specialises in your make and model of car, should you be unsure.