Like Wayne, much of my extensive roster of my freightcars have plastic wheels. I don't really have too much of a "gunk" problem, but I am replacing them as finances permit. I use P2K wheelsets. As Russ pointed out, rust is a greater problem...I used to have to clean off some of the axle tips and trucks regularily. I have also discovered the "truck tuner" used to ream out the trucks, it works great. I don't know which brand I have but there are several out there. I've never had any problems with axle length, the standard P2K wheelsets work in all of my Athearn, MDC, Accurail and Walthers freightcars. I haven't replaced any on Concor cars yet, I need to look. All of my cars run better after installing the metal wheels, but I think it is due more to the truck tuning than the wheels. I initially started to replace them mainly for appearances and as referenced earlier, the rust issue.