Upgrading Memory

See Install memory in your 27-inch iMac – Apple Support (UK)

Hello!

I have a 27" iMac 2020. I have two memory slots empty and 8 GB used. Can I add another 8 GB or do all 4 have to be uniform?

Yes, you an add another 8 GB of Memory for a total of 16 GB. When purchasing Memory for your specific iMac, you should only buy from Crucial:

https://www.crucial.com/.

Look for a 2 X 4 GB configuration (two 4 GB modules for 8 GB in total).

Other vendors' Memory for your specific iMac, such as OWC Memory, have issues in your model of iMac.

Jack

I upgraded my 2020 27" to 40GB of OWC (www.macsales.com) RAM. Below are screen shots that show, please not the arrangement I used. By the way, slot #1 is the top slot, slot 2 is right below slot 1.