My first 6v Optima was bought in 2007 - it is still in my 1954 Chevrolet Suburban.
Keep it charged.
Get a good 6v AGM battery charger/maintainer.
I have been told that if you let it sit, it discharges - it could be hard to revive.
$185 including shipping from dailysteals.com - $225 from most vendors.