Experts generally recommend synthetic transmission fluids for automatic transmissions, but drivers might want to check for manufacturer recommendations; these are set by the company that sells the car, not Allison Transmission. Unlike engines, automatic transmissions generally only need oil replacements every 30,000 miles or so.
Some automakers recommend transmission oil replacement every 100,000 miles or so since transmission oil stays in a closed system and is less prone to breakdown and contamination than engine oil. Synthetic oils oxidize more slowly than traditional oil, and they're able to maintain optimal viscosity through a broader temperature range, which can lead to smoother shifting and even slightly better gas mileage. Still, automatic transmissions are often tested with traditional transmission oil, so some drivers might feel more comfortable using it.