I have a Shimano RSX 8 speed rear derailleur and on a 20year old bike. It starts to play up and I would like to replace it. Since the RSX-Series is not produced any more, I was wondering with which model I could replace my old derailleur? Thanks everybody!
There's a number that would work. The first thing is to determine whether it's long or short cage (GS vs SS respectively in Shimano parlance). If you want something currently made that's a decent replacement of around the same quality, the Sora RD-R3000-SS or -GS is closest. It's nominally 9-speed but will work. For cheaper the current Claris R2000 will also work. If you wanted to get something nice for a bike you want to keep going indefinitely, RD-5701 would be good and is still made.