I guess that's the point. That CN Mikado, according to on-line sources, operated at a lower pressure than our little tank engine--only 180 psi(!), which seems low. No need to use the high temp/high-viscosity oil you guys are using.
Yet, that CN Mikado is superheated, and I'll bet the steam temperature at the valves can exceed 500 degrees F, during their climb up that long grade they have. Plus, that synthetic product was readily available for many, many years. Even if they were/are using that Inter-Lube valve oil, for superheated steam applications, that is still pretty thick.