I should point out that we are going through this evolution with our fleet of R44's.
If you hear a thud/bang that sounds like someone dropping a large heavy wrench/spanner on the floor in the back of the aircraft, then your sprag is in imminent danger of failure.
This is prevalent in our fleet which operate in +30c to -30c with up to 50 starts per day.
A failed sprag may seize but it will most probably run free = no drive.
Some of the sprags that we have stripped out ended up in 100 pieces
We are still monitoring the number of starts and are changing the fluid every 100 hours. So far the problem has been reduced by about 80% and we are cautiously optimistic.