Patrick, I have replaced all my plastic wheels (about 350 cars worth) with metal. about half were replaced with Proto 2000 wheels, the rest were mostly Kadee. I tried Intermountain, but getting the proper fit in the truck was a bit of a bother with them. The P2K and KD wheels too the least amount of "adjusting".
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By adjusting, if you find a tight axle, use a reamer (available from Micro Mark or your LHS) to add a little play to a tight axle fit, all to reduce friction and decrease rolling resistance. And yes, you don't have to replace the whole truck, just the wheel sets.
I did find a quick tutorial covering the basics .....
I other thing I do is paint each wheel set with rail brown or rust before I replace them. Saves me a step later on weathering.