Really difficult to find an officiale answer… I think this is the best place now to ask 
Memory “A-Tech 64GB 2x 32GB 4Rx4 PC3-14900 DDR3 1866 MHz ECC LRDIMM Server Memory RAM” are compatible with Z420?
I found L version too “Samsung 32GB 4Rx4 PC3-14900L DDR3 1866 MHz 1.5V ECC LR LRDIMM Memory RAM 1x 32G”.
What do you suggest?
Both options are a Load reduced memory, which CPU does the Workstation have? it has to be an E5-26xx for the load-reduced memory to work.
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E5-1660 V2.
Will be ok?
No, it won’t be OK as I said it needs to be a E5-26xx CPU
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I use pcpartpicker to check compatability.
4Rx4 L type ECC registered memory. 256GB confirmed working. I have several Z420. CPUs are 2680 or 2690 or 2696, all E5 V2. 256GB works. But I decided to go back to 128GB. 256GB runs very hot. Need good cooling for memory.