Bearings are usually eaten by the way many R/C car modelers run their engines. The fuels used have too little oil, and the engines are leaned to within an inch of their lives, trying to get every bit of RPM out of the engine.

If the bushing in the rod is spinning, then there's not enough lubrication getting to the lower end. Either the fuel doesn't have enough oil, or the engine's being run too lean. Gearing can be a factor if the engine's turning extremely high RPM. Run to hard or too fast, and engines don't last too long.

If low-end torque isn't a concern, then go with the on-road engine. The sacrifice some low-end torque ability because they're ported for high-end RPM.

For fuels, O.S. states that oil content of NO LESS than 18% be used. We'd also suggest going with a fuel that has castor. That gives a LOT of protection to the engine. There's a flip side to that, though. Fuels that help preserve engine life also don't clear as well when going from low to full throttle. Racers hate that, and so use low-oil fuels to get cleaner throttle action.

The thing to remember is that the people whose engines don't last too long either totally don't know what they are doing, or are pushing them very hard. That's why you see a lot of broken engines in racers' hands....they push and push and push.