Dave,
If you do replace with a synthetic seal from a QF kit, don't throw away your old seal. You might find that the compression tube is not perfectly round and a syn seal doesn't perform as well as a leather seal. Surprisingly, I've never had a completely trashed leather seal. I've cleaned them off and scuffed the edge a bit with coarse sandpaper then soaked for a couple days in Neatsfoot oil. You're not trying to sand off any of the leather only opening up the fibers so it can absorb the oil. Then wipe them off really well and reinstall. You'll be surprised that rejuvenating leather seals like this works wonders. The "forgiving" nature of the leather seal lets it conform better to tube imperfections.
I actually believe that for lower powered rifles leather seals are better than synthetic. Where synthetic shines are on late model high power/super magnum rifles with precision made tubes. Only then do you really reap the benefits of a synthetic seal. I've seen burned or blown through synthetic seals but I've never seen that on a leather one. Granted most of that is because typical older rifles with leather just don't produce the FPE of newer supermags (and less likely to get detonation hence a blown seal). By the way, I've had great success using buttons made from pop bottles on the back end of the piston. I use a sinlge hole punch to make them and then clean the piston with acetone and glue on using gel superglue. Then you just sand to fit. Really does smooth out the firing action. Old school does work!
Just my thoughts.
Andy Wong aka larspawn