[RichH]

Spectra is a very low stretch fiber (also used for boat ''rope''. The addition of spectra fibers to vee belting adds significant strength and lessens stretch. NAPA can get them but only by special order.
As regards the black powder/debris, another possible cause is the new sheeve has a different profile (more open or more closed vee angle) than the original and that is wearing the belt by chafe ... if so it will stop when the belt ''resizes'' itself to the new sheeve. But youre going to have to readjust belt tension occassionally until the belt ''settles-in''.

Mack Boring is adamant because Mack Boring sells "Yanmar" belts .... plain vanilla, rebranded (made by others), old style ''balata'' belts. They also sell genuine "Yanmar" filters (made by others and ''private labled'' for Yanmar). Think about ''mark-up'' and margins. ;-)