I would imagine oil changes would be less frequent. NG burns so clean the oil would not get dirty so quickly. My understanding the oil doesn't break down but gets contaminated from the carbon that's generated from gas and diesel.
You never hear about the maintenance costs of a NG engine, i know diesels are expensive with all the filters and what not, and with the anti polution stuff that needs to put on them Im suprised that a NG engine is so much more expensive??