Is this a true "Dichroic" head or a "Multicontrast"?
Dichroic filtration is usually given in increments of (M)agenta, (Y)ellow and (C)yan... from values of 00 to 170 - 200.
Multigrade heads are 00 - 4 or 5 -for corresponding grades, using Multigrade paper.

I don't think "fading" is much of a problem - athough I have heard of filter replacement.