Optiplex 990 Max RAM with A24 BIOS?

The basics specs of the computer will list what ram configurations are possible-- not the bios.

Manuals for all formats-- Support for OptiPlex 990 | Documentation | Dell US

yup, base specs are 16G, (4)x4G sticks, but several folks online have reported for years that (4)x8G sticks work, particularly in bios builds after A05, and I'm looking for more information. - specifically If I'm going to try that unsupported hardware change, what specs should I aim to find?   

None of the RAM that folks have reported working with appear to be available anymore at a vaguely reasonable cost, I *think* I've narrowed it down to looking for 8Gb 1333MHz DDR3 PC3-10666 DIMMs, but wondering if anyone has any success with anything currently on the market, or there's been any documentation from Dell about the change, or anything I'm missing in specs.

Here's some 990's with 32GB RAM:  https://www.newegg.com/p/pl?d=Optiplex+990+32gb+ram 

Just one 32 GB RAM kit:  https://www.amazon.com/Optiplex-Desktop-NON-ECC-PC3-10600-Tech/dp/B00VO5576M/ref=mp_s_a_1_4?dchild=1&keywords=optiplex+990+32gb+ram&qid=1611019798&sr=8-4 

Screengrab from https://www.dell.com/community/Desktops-General-Read-Only/32GB-RAM-in-an-Optiplex-790-990/td-p/3880625 

Even when users find that their Dell PC can exceed specs in some ways, Dell typically doesn't update documentation/manuals/tech guides.  That stuff remains back to the time of original manufacture.

32GB RAM won't be the cheapest.  Most brands are good.  KVR (Kingston Value RAM) has caused problems in some Dell's.  Some seem to take it and some don't.  It's what I've seen from being on this forum almost 3 years.

Though there's been no new BIOS versions for the 990 for quite some time, it is wise to have the latest BIOS before updating hardware.