There are three different orfice size jets...
There are three different orfice size jets for this stove which are designed for different fuel types. Can I use the smallest orfice jet (JET27) for all different fuel, so I don't have to change jet when switch fuel type?
This would be nice but there isn't a one jet for all fuels. This is also the reason they give you different jets for multi-fuel stoves.
The smallest jets #'s 27 & 28 are for kerosene/paraffin. Since kerosene develops most of it's BTU's from pressure, the smaller the jet orifice, the more pressure is produced inside the tank. #35 is for white gas/petrol which develops pressure much more easily, & #45 is for compressed gas canisters which are already pressurized. Therefore you will have to change jet's when changing different fuel types for optimum performance.
Case in point, it will take @ least twice as long to boil water burning white gas using the #27 jet as opposed to using the #35.