In the field I power mine with AA Energizer Lithiums I buy in bulk from the local Sam's Club. One set went 2 weeks in Colorado last year, however there's an important thing to remember if you choose to use the longer lasting lithiums in the 60CSx. Sometimes the unit will show the opening page on start up and then power down the first time you power up with a fresh set of lithium batteries. This apparently is a common problem with the unit as there's a lot of chat about it online. Good news though, there's a quick fix if it happens to you.
BEFORE you place a fresh set in your GPS unit, discharge them a bit by using them in a flashlight or similar device FIRST. Simply put them in the light and turn it on a few seconds, pop them out and place back in the 60CSx - you should be good to go plus they'll last a lot longer than standard AA alkalines.
FYI: The powerdown problem does not occur when using alkalines. I've been using lithium for about 3 years now with zero performance problems, however, occasionally I will have to implement the prodcedure just described.