Does it smoke when hot/cold or both?

The second number, i.e. 40 is the viscosity of the oil when hot, so it isn't that bad just slightly thinner than a 50 weight oil, however the 5 is the cold rating which obviously is a lot thinner than say a 10 or 15 weight.

Remember these are old engines and don't have the tolerances of modern engines and so it's too thin really, Ford recommended 15W/50!

Martin