As an example we had to use ancient mineral oil on the g3 apc's In iraq instead of the fancy new synthetic stuff the factory recommended.. simply because it got to thin in the heat.
If the high temperature grade is too low (e.g. a 10W/30), then the oil will be quite thin when warm, and this is what you get from the /50-grade: High temperature viscosity.
But you are right: Viscosity matters since it is essentially the viscosity that gives the greasing between moving parts, even under pressure.
I think a bigger problem is what the engine has been running on before - the engine I have in my old Espace J117 has a bit of sludge built up inside. It is clearly visible when removing the rocker cover as it sits everywhere inside the head. Sludge like that can cause problems if it is released and blocks an oil channel somewhere. The synthetic oil will not produce sludge, whereas the mineral oil will always.