oil will only attract and retain dirt and dust. And regardless of the viscosity originally, it will most definitely add drag as time goes on due to the stuff it collects around the layout.
I have used (but no longer) dry teflon powder for break-in on new rolling stock (not engines). It lubes while the plastic and metal are seating to each other, and eventually goes away except for a minute amounts that buries itself into the plastic. It doesn't attract or retain dirt or moisture and is plastic-safe. However, it is very messy and I found the benefit of using on my new replacement wheels was zip. It helped a lot on stock Athearn. MDC, etc wheelsets. But once I got rid of those and went to higher grade wheelsets it was no longer needed.
Decent modern trucks are made of delrin and other inherently slippery plastics and most decent wheels have highly pointed ends that reduce the contact area. once the metal axle and plastic have conformed to each other, lube is really not needed. If you are using the stock wheelsets, just changing the wheel/axles using the same truck frames can make a huge improvement, even without any lube.
if it is metal-on-metal, lube is however required, but a very, very light lube only. And a lot less than you would expect. For those I used a Slik 50 w/teflon spray. (the teflon is actually useless within oil.) But the reason I used it was that it was/is plastic safe and extremely lightweight (low viscosity). There are tons of model-train-specific lubes out there, LaBelle is a major name and they'll work fine. Again, less is best.
