I'm not in the heating trade but I have a Grant 36/46 that we downgraded to a smaller jet. The manual says it's rated at:

Nozzle = 1.0, output = 36kW, Net efficiency = 101.5%
Nozzle = 1.2, output = 41.5kW, Net efficiency = 98.5%
Nozzle = 1.25, output = 46kW, Net efficiency = 97%

The factory nozzle is the middle one - 1.2

Oil boilers don't modulate the flame size like a gas boiler so when the burner is lit it's allways burning at full power. Once the burner fires the output water temperature rises until it meets the setting on the boiler dial and then it switches off (even if the stats are still calling for heat). The output temperature starts to fall gradually and the burner eventually relights. The boiler therefore cycles on and off so that it's average output matches the setting on the dial. If you turn the dial up the on/off ratio increases. Turn it down and the on/off ratio decreases.

If your boiler cycles on and off too fast that may suggest it's too powerful for the heat load (can be other causes so don't rush into it). Fast cycling is inefficient (like stop start driving in a car) and if changing to a smaller jet reduces the rate at which it cycles it improves efficiency.

If you fit a jet that's too small the boiler may not cope in very cold weather.

The table above also suggests efficiency increases a little when the jet is reduced for other reasons - probably because the flame is smaller in relation to the size of the heat exchanger (a greater percentage of the energy goes into the heat exchanger and less up the flue).  

PS: It may seem odd that the efficiency can be greater than 100% but I believe that's down to the way efficiency is historically measured. The products of combustion contain hot water vapour and years ago this was wasted and ignored when working out how much energy the oil contains. These days the energy in the water vapour can be recovered (eg condensing boilers) but the way net efficiency is calculated hasn't changed. At least that's my understanding of how figures over 100% occur.

Just to make it perfectly clear, you cannot just pop out and buy any old nozzle you like.

The may be a range  of 1-3 that will be suitable for the burner/boiler