My '98 has the original battery in it and since it's been sitting a lot I thought that for sure it's going to go dead on me at some inappropriate time. I got a costo special size 48 with the vent tube and installed it with no issues. I used one of the Black & Decker cig lighter chargers from walmart to keep the clock and whatnot alive while the swap happened.
Anyway, I took the original to the local auto parts store (which didn't have the size 48 battery when I looked yesterday) and they tested it and said it's perfectly good. They just used some sort of small hand tester, and I don't know how much of a load that really puts on the battery.
People say the OEM batteries suck, but this one is sure a good one. I'm going to give it to my dad to use in his tractor and we'll see how long it lasts.