There are gotcha’s in this latest main SL Viewer 3.8.4, in the mesh upload. You can use more than 8 materials now. Gaia Clary has changed AvaStar to export more than 8 materials. But, the announcement of that change comes with a warning: don’t.

Also Drongle McMahon commented on the problem and what happens when we exceed the allowable polygon limit per material for mesh items. See: Second Life’s Limits.