he rightly said ,
although CNG is a gas but it consists of oil fumes
yes you have to get your cng tank + kit serviced
CNG TANk service
they invert the tank ( nozzle lies on the bottom)
all the oil in it comes down due to gravity , then they slowly open the nozzle , the pressure inside tank from cng filled forces to let oil get out of nozzle , as all oil get out of cylinder after they they release small amount of cng out of nozzle making sure tank don't have any oil,after that they fix back again
CNG kit service
its due to cng natural property it consists petrol fumes , so yes CNG Kits also get oil as well inside it
they open it , remove all oil , service with kerosene oil making sure all parts inside are moving freely , then fix back again
+ they even do tappet + distributor + spark plug cleaning + point cleaning
+ Flow of cng over rpm over hose pipe + cng kit settings
1 year for cng kit service , 2 years for cng cylinder service