This subject has been covered previously, in the 'Jetranger - Longranger' thread if I remember correctly.
Yes, the Allison (Rolls Royce) 250 Series engines are very prone to carbon build-up if certain oils are used. By this I mean the make/model of the oil rather than the mil-spec. For example, my early experience tought me that using Aeroshell 500 in these engines virtually guarantees substantial carbon build-up, a change to Mobil Jet Oil II completely reversed this situation. I've very little experience with other oils but have heard that Mobil Jet Oil 254 is also very good.
handbag: Which oil were you using prior to BP2380?
meloni: The 2 min cool down time is a requirement, not just 'good practise' and is required regardless of which oil is being used.