I would suggest that mixing oils in a turbine engine is not a big deal. In fact, sometimes it is the recommended procedure. The currently accepted method of oil type change-over is the top-off method, which means you simply add the new type of oil to the old oil. Eventually, you end up with a oil system completly full of the new type of oil. Complete change-overs are not recommended because they spike the additive package which causes plugged oil filters. The "little black balls" were bits of debris left by the old oil, and dislodged by the detergents in the new oil. 2380 allways left some carbon in the oil system. 254 is a more detergent oil, so when it is added to 2380, it will cause some filter clog lites to come on. The new oils, Exxon 2197, and Mobil 291 promise to be the best turbine oils yet, but they have a strong detergent package in them, which when added to an old oil system, will be bound to plug some filters, but that is only because they are cleaning a dirty oil system, not because they are incompatable with the old oil.
On the other hand, Sundstand warrenties their IDG's based on oil brand/type. So mixing of oils there should not be taken as lightly.
Please check your companys procedures on this issue. It makes for some interesting reading.