The Rolls Royce Customer Service Letter (CSL 1208) recommends the following -
In order to minimize the shedding of previously accumulated carbon/coke build--up, it is recommended that the switch to an HTS oil be accomplished only on new engines or engines that are newly overhauled or repaired where the hot oil wetted surfaces have been cleaned. In service engines should be changed over gradually during operation by the ”top--off” method as described below.
If a switch to HTS oil is desired, and the engine is not new or freshly overhauled, it is recommended this change be accomplished gradually using the ”top--off” method. The ”top--off” method decreases the likelihood of shedding of previous carbon/coke build--up. In the ”top--off” method, the HTS oil is added as oil is consumed by the engine or a quart of oil is removed at 10 hour intervals and replaced with the new HTS oil. At the next scheduled oil change, normal oil change can be accomplished utilizing HTS oil. When the engine is returned to service, watch for impending bypass indications, and check the scavenge oil filter and engine oil filter on a regular basis for carbon/coke deposits. These indications could be a sign that carbon/coke, which was formed during previous operation, is being dislodged and circulated through the system. The accumulation of dislodged carbon should diminish quickly. Maintain filters as required.
The use of an HTS oil does not eliminate the requirement to run the engine at idle for two minutes in order to reduce oil wetted turbine skin temperatures and stabilize turbine wheel rim temperatures.