Some history: the reason for AD-2002-21-16 (now rescinded and replaced by AD-2005-01-14) is a certain influential but misguided person was telling customers and maintenance shops to blow out the oil cooler and oil lines with shop air and to rotate the prop backwards with the oil filter OFF every oil change!! (to pump all that dirty oil out of the engine)
He believed this was the best thing to do during an oil change. He was wrong and 3 inflight valve failures resulted. Basically the engines were experiencing a dry start every 50 hours.
The FAA demanded strong action to prevent this from happening again. The first AD did not allow the prop to be rotated backwards, even an inch! Also the purging had to be done every oil change!
Of course this is an over-reaction, normal turning of the prop backwards does not harm the engine and normal oil changes will not introduce enough air in the lifters to cause any harm.
So now with this knowledge we know that as long as you follow the Line maintenance Manual oil change procedure and do not introduce large amounts of air into the lifters you will be fine. We also know that the small amount of dirty oil left over after every oil change in not to be worried about, you will get it next time! (and if you oil is really that dirty you need to change it more often)
This is a very short version of all that took place back in 2002, we also had people starting engines for the first time without any purging, using anti-dispersant (AD) oil, diesel oil and silk thread on sealing surfaces so the general knowledge of the Rotax engines has greatly improved since then.
