When I was on the DC-8 with Pratt engines and the 747 classic,also pratt engines, it was a limitation out of the manual not to use engine anti -ice above 10 TAT. The L-1011, rollers obviously and the A-300, GE's, had no such limitation. There was always conflict between the pratt guys and everyone else about the 10 degree limit. There is no magical phenominon(sp) about 10 degrees, just good common sense.