Assuming the air compressor has an oil content of 5ppm, the calculation for its monthly fuel consumption based on a displacement of 16.2m3/min is as follows:

At standard conditions (1 atmosphere, temperature 273K), the weight of 1m3 air is 1.276kg. Therefore, the oil content per cubic meter is:

5ppm ÷ 1,000,000 × 1.276kg/m3 = 6.38kg × 10^-6/m3 = 6.38mg/m3

This means that each cubic meter of air contains 6.38mg of oil.

The fuel consumption of the air compressor per minute is:

6.38mg/m3 × 16.2m3/min = 103.36mg

The fuel consumption per hour is:

103.36mg × 60 = 6201.6mg

Considering the compressor works 24 hours a day, the daily fuel consumption is:

6201.6mg × 24 = 148838.4mg

For a working month of 25 days, the monthly fuel consumption is:

148838.4mg × 25 = 3750.96g = 3.75kg

Conclusion: Based on the 5ppm oil content assumption and a displacement of 16.2m3/min, the monthly fuel consumption of the air compressor unit is approximately 3.75 kilograms.