NBR (aka Nitrile/Buna/Buna-N) is the most common carburetor o-ring material. It is fairly good for use with pure gasoline, but not so good when any amount of ethanol is present. It most commonly has a working temperature range of -31 to +230° F. It is what is supplied by OEMs and what is found in the vast majority of aftermarket carburetor rebuild kits.
FKM (aka Fluorocarbon/Viton) is by comparison a relatively rare carburetor o-ring material. It is excellent for use with pure gasoline, ethanol blends, and pure ethanol. It most commonly has a working temperature range of -4 to +410° F. It has a much greater life span than the above, is therefore what we supply in our kits, and is what 99% of the o-rings we sell separately are made of (if they are not, they will not be described as FKM in the listing). As an example, we rebuilt a rack of BS30 carburetors with FKM o-rings. When they were taken apart again to clear clogs after having been exposed to 10% ethanol fuel for 7 years, we found all o-rings to be in reusable condition.