Internal diesel injector deposits (IDIDs) are now a well understood phenomenon and a standard test procedure has been partially developed and approved by CEC. The engine test procedure has been approved for simulation of sodium soap deposits by dosing the test fuel with a sodium salt and DDSA, whilst lacquer amide deposits simulation by dosing the test fuel with a low molecular weight PIBSI is still under development. The solubility of these contaminants in the base fuel should be reasonably constant to achieve consistent results. With the introduction of diesel from varying sources, this study focused on the effect of prEN 15940 compliant Gas-to-Liquids GTL diesel, very similar to HVO, on IDID severity across two different engine platforms, and the response of a modern deposit control additive. The test results showed that with the same level of contaminants, IDID severity did differ significantly when comparing GTL diesel to a mineral diesel reference fuel. The IDID effects also differed across engine platforms. In a previous study, it was found that the effect of zinc solvency in the base fuel had a dramatic effect on the CEC F-98-08 DW10 injector nozzle fouling test results. For the IDID test procedure used in this study, the effect of base fuel solvency was less severe on IDID formation. It was also found that some degree of nozzle fouling does take place with typical IDID contaminants. The deposit control additive tested proved to be effective in preventing both IDID formation and nozzle fouling for GTL diesel and mineral diesel. In conclusion, it was highlighted again that base fuel solvency can influence the results of a test procedure where contaminants are dosed into the fuel, and these effects need to be considered where such fuels are tested.