In the instructions with the Lionel 4-4-2 and 2-4-2 steam engine starter sets, they recommend pulling no more than 10 cars. And those engines have a slightly bigger motor than does the 0-8-0.
Though my 4-4-2's will certainly pull more than 10 cars, I'm guessing the advisory might be related to the smoke unit (as well as prematurely shortening the life of the motor). The longer the train, the higher the voltage you are putting to the track, and therefore the smoke unit is going to get hotter, faster.
My oldest 4-4-2 is now 30 years old with the original motor. But I had to replace the smoke unit after around 13-14 years. That indeed could be normal, but I also suspect I might have shortened its' life by routinely running longer trains. Even with the puffer type smoke units, the Lionel Premium Smoke Fluid is a huge improvement in smoke output for these sorts of engines, not normally noted for such.
Needless to say, modern types of rolling stock with fast angle wheel sets are going to be less stress for the locomotive than postwar style trucks that typically have more drag. Of course, keeping those clean and the wheels lubricated certainly helps.
Your layout configuration is probably also a factor. Level track is better than elevated up and over types of designs. More curves, as with a figure-8 are also more work for the locomotive to pull a train versus an extended oval. And probably the 36 inch diameter curves of FasTrack is easier on the locomotive than the tight 27 inch curves that I use.