From Rolls Royce C20B Customer Service Letter CSL 1178, which was issued back in 1993. "It has come to Rolls-Royce’s recent attention that a number of Model 250 operators have adopted another engine manufacturer’s cool down procedure intended to reduce coke/carbon formation in the turbine area.
We understand that this procedure calls for the hot engine to be motored with the starter immediately following shut down to provide additional oil flow for cooling the turbine bearing/shafting area.
Operators must understand that this procedure is not recommended by Rolls-Royce and can accelerate rim cracks of the first and second stage turbine wheels due to the thermal shock of introducing a flow of cool air over the turbine wheels.
Operators wishing to reduce coke/carbon formation in the engine should follow Rolls-Royce’s currently published recommendations:
Use MIL-PRF-23699 Rev. G or subsequent oils.
Cool engine for two minutes at a stabilized idle prior to shut down.
install STC approved scavenge filter.
Change oil at recommended intervals.
NOTE: This CSL is not intended to change the currently published procedure for restart shortly after shutdown by cooling a warm engine below 150°C prior to light off through motoring.