Yes, I replaced my belt due to shredding on flight 13 and at the same time installed the idler pulley from the MX400. The MX400 pulley has flanges on each side that help to ensure the belt remains centered on the tail drive pulley.

I have not decided on whether to use any belt dressing on my new belt, but it sure won't be WD40. I won't use WD40 for anything. I intend to see how things go with the new pulley and the try something else if that doesn't correct it.