As above,
My first belt lasted just over 6K miles- no grease, but started shredding long before that. It didn't destruct, just stretched, and warped/twisted out of shape. I replaced it with a belt procured from the local bearing supplies (they had to get it in), and it's done ~3K miles since then, with no sign of distress at recent checkup, at which time was a bit tight, so backed it off some (still no grease).
Replacement belt is polyurethane reinforced, 'Optibelt ' brand (Made in Ireland), and I bought in a spare a couple of weeks ago.
There are several different manufacturers. AT5-420 x width (mine= 10mm )
Picture shows new-new belt overlaid on old one, and second pic shows the deflection I originally had with the 1/2 twist (SRM/Synchroflex), but I think when I backed the new one off, I did so it it was just clear of the cover screw boss (underside).