You need the exact oil viscosity it says on your oil filler cap.

Define "topping up". 

I would avoid mixing oils personally. @Pulp-Friction is right: use what your oil cap says.

When the engine is cold, "half hash mark” level is fine, … you don’t need to add oil right now - oil expand with temperature. Why don’t you re-check level when oil  is hot. At that point oil level should not exceed beyond upper level of hash mark.

Anyway, as long as indicated level is on hash mark - not above and not below, … your level is fine!

F.S.

… btw, when changing or adding oil: … just use what’s recommended, what’s written in the owners manual!

My engine has 5w-40. But the level on the dipstick when cold is at half the hash mark. So i plan to add with 0w-40 or 15w-40