https://www.optimabatteries.com/en-us/support/battery-basics/deep-cycle-battery
http://www.bruteforce.com/index.php/default/gba8vhdc.html

You did not ask (sorry), but, most likely you will not need a deep cycle 8v battery in a 6v system, unless you have have high-draw 6v devices, and/or, you plan to use 6v devices with the engine/generator not running/charging. Any 8v battery will spin the 6v starter faster. 6v incandescent bulbs might be brighter, but incandescent bulbs might wear out faster.