Adding a black WD 4T question

Quick newbie question.
So I have a Drobo 5N with these installed.
Bay 0 = WD Black Drive 1T
Bay 1 = WD Red Drive 4T (one month old)
Bay 2 = WD Red Drive 4T (one month old)

So im kindof worried about longevity and heat if i install another spare WD Black Drive (one month old). At the most 3 users at the time and 75% of the time the drobo will be “disk drive spindown”. They are all under warrenty but ofcourse the info is not. So is it wise to install another black drive in the drobo or just get another Red drive and play it safe? I know its ok to install WD black drives but anyone have any issues with installing all black drives with heat and longevity?

I would add another WD RED drive, but if you already have spare WD Black drive then AFAIK it’s OK to add it to drobo 5N.

Take a look here:

I have to get rid of my last WD Green drives and replace them with WD RED drives …