A reader asks: I’m using 100% biodiesel derived from used vegetable oil in my van’s diesel engine. I’m told that I have to change the engine oil more often when using biofuel. Why is this?

Our answer: There are several reasons as to why oil change intervals must be reduced when replacing diesel fuel with biodiesel.

Lower Fuel Energy And Deposit Formation

The main concern with biodiesel is that it provides reduced fuel quality. The simple fact is that biodiesel delivers less energy than regular diesel fuel. This means that combustion is less complete than with regular diesel. This brings a greater tendency toward fuel dilution in the motor oil and an increased incidence of engine deposit formation.

Vegetable-based fuels also have a higher tendency to form deposits on fuel injectors. Fouled injector spray patterns can lead to increased fuel dilution in the motor oil.

Biodiesel Oxidization And Moisture Attraction

There is also a very high potential for increased engine deposits when biodiesel has oxidized. The quality of biodiesel processing and storage practices is absolutely paramount. Incomplete processing and bad storage practices can lead to a myriad of problems. Oxidized fuel can contribute to poor combustion, reduced fuel economy, engine deposits, corrosion, fouled fuel injectors and reduced starting performance.

Biodiesel attracts moisture which can lead to corrosion and reduced performance. This is another reason to use the best possible storage practices.

All of these factors can contribute to faster degradation of engine oil in bio-fuel-powered vehicles.

How To Determine An Oil Change Interval When Using Biodiesel

With all of the variables at play, it is very difficult to determine an oil change interval. Oil analysis is a cost-effective way to determine when to change oil. One could be changing oil too late or too early. Oil analysis can offer a snapshot of the condition of the engine and the motor oil. Analysis shows whether items such as as wear metals, fuel dilution, moisture and coolant are elevated to dangerous levels.

Advantages Of Using Biodiesel

There are advantages to using biodiesel. Some studies suggest that biodiesel can offer improved engine wear control. Biodiesel has excellent lubricating properties which can extend the life of fuel pumps and injectors. Lastly, pure biodiesel offers very clean emissions compared to regular diesel exhaust.