At what (if any) engine temp. should I ADD oil?
 

Adding (not changing) oil. Should the engine be cool for this? Does it make a difference?

Thanks.

Doesnt matter. Since you need to check the oil when the car is warm I think it makes sense to add then as well.

I've found that this works-car must have three facets covered:
car must be on level ground,
car must idle at around 800 or so rpm,
oil should be "at temp" say 190 degF.

Then fill to the mid mark on the oil dip stick . If you fill to "full" , there is a chance that when you run even hotter than normal, the expansion drives the excess oil to the intake and you note "apparent" high oil consumption. Don't fall into that trap.

---Wil Ferch