There is a surprisingly high percentage of idling as trains can often coast for many miles once up to speed. The effects of idling being offset against the high power demands of accelerating away from a stop. This sounds a bit like the "Italian tuneup" advice often given to boaters with smokey engines, a high power run on deep water to get everything up to correct temperature for about half an hour and alls well again. So perhaps a little synthetic is fine providing you can get it up to full working temperature once in a while?
That said, I ran my nearly new Lister LPW3 on Mobil 1. It was slightly undersized and over propped for a 62' nb, so no issues with getting it up to temperature, with the added bonus that it would idle smoothly from start up in sub zero temperature because the thinner oil didn't "gum up" the governor linkages which are internal to the engine.