it has been mentioned before to avoid engine flush as this could spell disaster for a standard engine let alone a performance one.

i dont know about rs engines, but i had a similar issue with a 2.6 audi coupe. it was terrible after an overnight rest, i thought the cams were about to bounce out, but i got told as they were hydraulic followers they drain of oil and the first couple of seconds was before they filled up again. i was using semi synthetic oil and swapped over to fully synthetic and it seemed ok. im not advising, just what i found personally.