Yes ....you can use a 9spd chain with an 8spd cassette, the difference between 8 and 9 spd chains is in the width, with the 9 spd chain being slightly narrower. You cannot use an 8 spd chain with a 9 spd transmission though. Provided that the chain does not fall into the gaps between each sprocket the only slight disadvantage that you may incur is that the shifting may not be quite as crisp. It is unlikely that you will notice any difference. Generally speaking....like for like quality and link length....the 9 spd chain will be slightly lighter as there is simply less metal in there. Personally, and regardless of all the sales hype from the industry, 8 spd chains are stronger....quality for quality. There is no substitute for metal. I use an 8spd set up for my " training in all filthy conditions " bike, as it simply outlasts and outshifts my 9 and 10 spd transmissions. Frankly the 10 spd is dreadful in mud and grime.