Leeda Silicone Mucilin
This stuff is superb for keeping your fly line sitting high in the water on those fast rivers. I use this all the time now and before I start to fish I will treat my fly line and if fishing Dry Fly treat most of my leader also. This helps with float-ability and also helps with lift of when you have to be quick to change direction and cast to a rising fish.
Over a longer periods of time this will crack many fly lines, but I buy cheap floating lines for the river and replace them almost every year, at the end of the day how far do you need to cast on the river? If the lines does its job and normally I pay no more than £10 for my floaters and throw them away at the end of the year.
Excellent product that has stood the test of time and has been around for years and years.