For my Excal I just keep the string and serving waxed. I was using some brand of rail lube or another in the beginning but was getting build-up, etc. Also seems like when I put something down on the rail it attracts every speck of dirt, weed particles, etc. and probably ends worse off than before I apply it. I haven't noticed any problems with just keeping the string and serving waxed up with regular string wax. I heat it up with a hairdryer, or partially melt the wax in the stick with a lighter then apply it and rub it in really good. I'm not even totally sure what Excalibur recommends for the rail if anything. My Horton recommended Horton rail lube which attracted dirt like a magnet.

I just looked up on Excalibur's owners manual and it says for their bows to only use serving wax and to not apply anything directly to the rail. I was thinking that was the case. I would go with whatever your CB manufacturer recommends. If you use anything with a solvent in it, it may cause premature wearing of your string/serving.