a couple of questions for a restomod

[1969_Mach1]

I was under the impression that 20w-50 is the oil the engine was built for and can be used from temporate to sub tropical temperature zones. I use GTX 20w-50 in my older cars all year round here.

I have only seen a couple of temp vs. viscosity charts. Both listed 20w-50 for temps no lower than 32 deg. F. I don't know if there is a more complete and accurate source.

Best Regards,
Mike