Shortened oil change intervals can be expected when using a biofuel and fossil fuel mix above ____________.

When using a biofuel and fossil fuel mix, shortened oil change intervals can typically be expected when the biofuel content exceeds a certain threshold. In this case, the selection of 20% as the threshold makes sense because studies and practical applications have shown that beyond this percentage, the properties of the fuel, particularly its combustion characteristics and the byproducts it generates, can lead to increased contamination of the engine oil.

Biofuels often have a different combustion profile compared to traditional fossil fuels, resulting in an increase in the production of carbon deposits, soot, and possibly other contaminants that can degrade the oil's properties more quickly. This means that as the biofuel content increases past this point, the oil can become less effective at lubricating and protecting engine components, necessitating more frequent changes.

Thus, when the mix exceeds 20%, the impact on oil degradation could be significant enough to recommend a reduction in oil change intervals to maintain engine health and performance.