Its better change the Oil to some more Viscous as check to shift to Petromin Oil or Havoline or someone having rating of SAE 20W-50 it will work better and can contain the Oil Decreasing mostly the Drain Nut have the Seal Worn out and causes the Leakage of Oil.
Its better to lift the car and check underneath for Leak if have otherwise shift to other Oil and check, CNG have some relationship for the Oil Decrease as CNG has higher burning point and if you are using oil have low Viscosity at 100 'C thn it will b decreasing so better switch to 20W-50.