If OELCHECK detects less diesel (e.g. 1.2%) and more biodiesel (e.g. 2.8%) in a diesel engine oil, this does not necessarily have to result from the use of pure biodiesel (so-called B100). Especially if the engine oil has been in use for a long time, such values are "normal" for today's commercial diesel fuel as it comes out of the pump.

Specification-compliant diesel according to EN 590 has contained small amounts of biodiesel for several years. Legislation has stipulated that up to 7% of biodiesel produced from renewable raw materials should be added to conventional fuel. Engine manufacturers have agreed to this 7% addition as part of the EN 590 standard. Modern diesel engines can generally cope with this addition without any problems. However, difficulties can arise if an engine oil is used for a very long time and/or the vehicle is frequently used in short-distance traffic. Our analyses have shown that a disproportionately high share of biodiesel is then found in the engine oil. Here, condensation of unburned fuel or blow-by occurs preferentially on the cold cylinder wall. These particles are discharged "downward" toward the oil pan by the piston stroke. This effect is particularly pronounced when high power is required from a "cold" engine (e.g. emergency vehicles). However, fuel also enters the engine oil in the event of problems with the injection system.

Conventional diesel has a relatively early start of boiling. It can vaporize at oil temperatures that often exceed 80°C in the oil sump when the engine is at operating temperature. The vapors are then fed back into the intake air via the crankcase ventilation. They thus enter the combustion chamber, where they are burned together with the fuel. Biodiesel, 7% of which may be contained in the fuel, enters the engine oil via the same route. However, biodiesel has a significantly higher boiling point. In contrast to mineral diesel, the biodiesel present in the used oil therefore evaporates from the engine oil far less easily or not at all. This means that biodiesel accumulates in the engine oil. The biodiesel added to the diesel in the refinery may still contain small amounts of "triglycerides". Triglycerides and other fatty acids or biodiesel components can polymerize with increasing concentration at normal engine oil temperatures and long operating times. This often results in deposits that are present as a kind of coating on all oil-wetted parts. Among other things, they hinder heat dissipation through the oil and the necessary oil flow due to filter blockage, for example. In addition, there is a tendency for coking to form at hot spots, e.g. in the piston ring grooves and at the piston crown. These can then lead to engine damage.

Conclusion: Since "normal" diesel fuel today contains up to 7% biodiesel, proportions of biodiesel are repeatedly detected in engine oil. In order to avoid the risks to the engine described above, however, the proportion of biodiesel in the engine oil should not exceed 7% if no manufacturer approval is available for higher biodiesel contents.