40 odd years ago I worked for a DC8-50 operator that experienced ice blockage of the inlet to the Fuel Oil Heat Exchangers on engines 1 and 4 on one particular sector. I don't recall if there were any power reductions as a result but the indicated fuel pressure on 1 and 4 used to drop late in the 9.5 hr flight. Investigation revealed impact icing on the fuel inlet end of the heat exchanger core. Part of the problem was the fuel loaded at HNL contained too much water as the tank farm capacity back then did not allow sufficient settling time before use. Engines 1 and 4 only were affected because the coldest fuel was in tanks 1 and 4 toward the end of the sector.
The fix was to reverse the oil flow through the FOHE. The original design had the hot oil entering at the fuel exit end and the cooled oil exiting at the fuel inlet which I recall was supposed to be the more thermally efficient flow but by reversing the flow the hot oil went into the fuel inlet end of the core and stopped the ice from building up on the FOHE core.