@Ser_Gtr , Bhai Oil in CNG Cylinder keep on increasing by few drops every fill-up, it all depends upon the milage done and the size of the Cylinder.
In my experience, my Daewoo which has a 60 liter (big) cylinder does not need any Draining after some 55,000kms on CNG, ok.
Now, about Small Cylinders, such as Kia Spectra's, it has Two 27 Liter Cylinders, Oil is collected @ double the pace of a 55/60 liter cylinder, thus it needs Draining quite early like 30k or 40k kms.
For CNG Kit, You can get it Drained and tuned up for better performance from a good CNG mechanic. Kit's Service should be at every 25,000kms, which can be dragged to 50,000kms if you dont feel any missing problems, my Daewoo has a Landi-Renzo, Un-serviced for 55,000kms
Do keep changing the Air-Filter every 4,000/5,000kms max. As the conditions of Karachi these days are too dusty.