The salesperson at the store was wrong. They are fine as a starting battery. They work well and will not be damaged when used as a starting battery. The other way around does not work; you can't use a battery designed only for starting as a deep cycle battery (at least not for very long!). Go to the Duracell site if you want to read about uses of their batteries. Keep in mind the Duracell group 31 is not a direct replacement for the 27F size that is OEM. You will need some mods to the tray, holder, and cable length to fit a group 31 in there. And, if you really only need a starting battery, the Toyota OEM is hard to beat for price and warranty. i have a fairly heavy use of my electrical system when towing and with a Waeco refrigerator, so the deep cycle battery made sense for me.