Diesel won't harm the pipe but will smut it more and it will not produce as dense a smoke as a paraffin oil will. if you're having a residue (a liquid film not just smut) left inside your pipe move the injector tube outlet in toward the exhaust stream a bit more, the residue is because of excessive oil flow (in this case add a second injection tube) or not a hot enough temp (moving it in closer to the jet blast as mentioned corrects this) to completely vaporize the oil, and a turbine EGT is defiantly well above the temp to do so.

as Lance stated, it's difficult to get two smoke trails much past the tail of the aircraft because of the jet blast from the two outlets being so close together. twin injectors does helps get better vaporization of the smoke oil (compared to the same amount of oil from a single tube) but it's hard to tell in flight a single from a double even on bifurcated pipes.

kc