Differences between Trent 800 and GE90
One of the reasons that interim procedures published by Boeing apply only to Trent 800 powered 777s could be differences in design between the two engines. (See Boeing 777 FCOM)
Fuel System, Flow Path:
Trent 800:
1st Stage Engine Driven Fuel Pump
Fuel/Oil Heat Exchanger (FOHE)
Fuel Filter
2nd Stage Engine Driven Fuel Pump
Fuel Metering Unit (FMU)
GE90
1st Stage Engine Driven Fuel Pump
2nd Stage Engine Driven Fuel Pump
FOHE
Fuel Filter
FMU
This means that on GE90 engines the fuel has already gone through the second stage (HP) fuel pump before it enters the FOHE, which will cause some rise in temperature
Oil System, Flow Path:
Trent 800:
Oil Pump
HP Oil Filter
Air/Oil Heat Exchanger
FOHE
GE90:
Oil Pump
HP Oil Filter
FOHE
Backup Gen Oil/Oil Heat Exchanger
That means that in Trents the oil has already part of the heat extracted in the air/oil heat exchanger before it makes its way into the FOHE.
No conspiracy theory required after all?