You can google on the nets and get the lowdown on the DOT numbers. Basically, for a FJR, you want to be using a high quality DOT 4. Valvoline Synpower is readily available and well-liked.
Back to the original question, the amount of moisture introduced into a can of brake fluid when it is opened and resealed is truly minisicule. (I computed it one time assuming a half-full, big 32-oz brake fluid bottle and using typical temps, humidities, etc. to be about 1/100th of a drop of water.) This is really the only problem with using fluid from a container that has been previously opened - it doesn't deteoriate otherwise. So I don't see any issue with using fluid from an opened can as long as you're sure the can is well sealed. And brake fluid is nasty stuff that really should be recycled but most people dump it down the drain or throw it in the trash - if you're one of these, throwing away good brake fluid to be ultra-conservative is hard on the environment. If you recycle like you should, then it's just a matter of money.
That said, I'd buy a container that is more/less compatible with how much you'll use in a small amount of time. IOW, I wouldn't buy the Costco size to save money and then open and close it over the next ten years. Each time you do, you're refreshing the air in the container which contains new moisture.
- Mark
h34r: down yer way, in a rock band, so quality wouldn't be an issue, an he might buy it :lol: 
