Brother why would you change the oil from 10W30 to 20W40 for a VTEC engine ? its a really bad idea.
Check the idle screw of CNG kit and adjust gas supply from it till your engine is smooth at idle.
Never let any mechanic touch your throttle body idle screw and slap him if any 'chota ustaad' wants to touch it with screwdriver.
Dont worry its a simple 5 mins job.