T410 RAM Issues - single/dual ranks

Hi,

I've inherited a T410 which came with 2x8GB RAM RDIMMS, in slots A1 and B1. The server was happy with this. Dual CPU configuration.

I wanted to add more RAM so I thought it simplest to add another 2x8GB RDIMMS. These came but there is a very minor difference, they are dual rank while the originals are single rank. The hardware manual says you can mix single and dual ranks, but doesn't give guidance about where to put them. It suggests slots A1/A2 and B1/B2 should be filled in this configuration for 'optimizer' mode

Every possible configuration of single and dual rank (each CPU has single or dual, split single/dual to have identical configurations per CPU etc etc, different slots) all give various errors either 'sub-optimal' or 'unsupported configuration'

If I use A1/B1 Dual rank and A2/B2 single rank it says 'unsupported, DIMM geometry should match A1/A2 and B1/B2'

If I use A1/A2 dual rank and B1/B2 single rank it says unsupported: CPU config should match

If I use A1/B1 dual rank and A4/B4 single rank it says 'non-optimal, something about a mismatch

Can someone please tell me what configuration of 4x8GB (2 SR and 2 DR) should be used for maximum support/compatibility? I understand an error might persist since technically geometry doesn't match, but if I could be advised which error is 'best' that would be good. Should I try to make each CPU identical or put all the single on one CPU and all the dual on another?

Thanks