I hear varying things about chain compatibility.
I have multiple cheap bikes with 6-speed freewheels. These have 5.5mm sprocket spacing, whereas 7 speed freewheels had 5.0mm spacing. Later cassettes 9, 10, and 11 speeds have even narrower spacing.
That's all well and good, but for my 6-speed bikes, should I be seeking out a 6-speed chain in order to work best with them? It seems to me that the wider chain might be better for some reason such as grabbing onto the sprockets easier or being less likely to fall between them.
It's getting harder to find 6-speed chains. Many chains claim they are compatible with 6, 7, 8 speeds or even 6-11 speeds.
I'm not asking about what is strictly compatible or what will work, because that is listed on the package. I'm asking more about what works BEST. What compromises are involved in running "higher speed" chains on wider-spaced freewheels/cassettes? Is there benefit to avoiding the narrower chains on older bikes?
EDIT: The wikipedia link provided below had the key information that "Shimano used/uses the same chain for 6,7,8 speed designs". So, if the package says 6/7/8 speed compatible, there is no compromise at all, because from the beginning (at least within Shimano), the chains were exactly the same for those designs anyway. So, chains which advertise "6/7/8" speed, should probably be believed.
This does not explain to me why some chains are marketed solely as "8 speed", however.
Also, Sheldon Brown's website says "using a chain one size narrower than standard rarely presents any problem. Thus, you can use a "9-speed" chain with a 7-speed or 8-speed system, or a "10-speed" chain with a 9-speed system. This is not the ideal approach..."
Now I know there is no need, or even benefit, to trying to find "6 speed" chains, because they are not any different than 7 or 8 speed chains. However, chains which claim to be compatible between between 6/7/8 family and other speeds such as 9, 10 or 11 are not telling the whole truth.