Plaster and lath is typically 780 lbs/yd3. A 8 yd3 load is about 2.2 to 4.4 tons (typical 3.12 tons). Mixed loads vary widely by composition and packing. Use the safe recommendation when schedule risk is high or the heavy portion is uncertain.
Plaster and lath is usually a weight-first decision surface. Treat volume as secondary after tonnage limits.