8GB RAM in a Late 2013 iMac.

[xraydoc]

I would recommend adding 2x4 GB or 2x8 GB for best memory performance. If 16GB looks like it wouldn't be enough they may as well go to 24GB so they don't have another 4GB module they would have to replace if they found out they needed more than 20GB.

Plus, there's a small performance boost when using matched pairs.

 

[AGamble]

Plus, there's a small performance boost when using matched pairs.

Do you have to match pairs?

So theres a downside to having 2x8GB and 2x4GB chips installed?

 

[Tjosansa]

Do you have to match pairs?

So theres a downside to having 2x8GB and 2x4GB chips installed?

I never heard that before!
ive got 2x4 and will add 2x8 later, for a total of 24.

 

[SaSaSushi]

Do you have to match pairs?

So theres a downside to having 2x8GB and 2x4GB chips installed?

No, those are matched pairs. (a pair of 8GB and a pair of 4GB)