Personally, I think Polar Pros are over rated and over priced.
I use Neewers and their weight is why.
See: http://forum.dji.com/thread-43168-1-1.html
Been using them for almost a year now and had no problems at all.
The X5 manual gives no specification, but the Inspire 1 Pro manual does.
"Filters must have a weight of 7-11g with those weighing 10-11g performing the best"