@all
generators are just engines, similar to the ones in our cars. just take care to ensure that you use a very good oil (any brand), keep the air filters clean, regularly clean the plugs on the petrol ones and ALWAYS make sure you have some ratio of anti-freeze in the radiator as antifreeze also contains rust inhibotors.
do note that CNG has an octane rating of around 130 and you MUST make sure your timing, air/fuel ratio is correct and your plug gaps are right. Most gas generators run extremly hot exhaust gasses so be careful not to fry your hand when working on the generators.
i'm using Petromin Superdiesel in all of my diesel generators,.. main reason is that it is a suitable grade for them and comes in 20L pails (ideal for large generators 40+ kva).
just take care of your generators as you would your cars & motorbikes and you'll be ok.