Hi,
Dsm 6.2 (DS3615XS) WORK to Virtualbox.
I managed to run dsm 6.2 (ds3615) on Virtualbox, be careful that version 6.2.1 does not work). It will be necessary to download the good spk on the syno download center.
You must first configure grub (the mac address must match the one entered in the vm), then convert the .img file to .vdi (VBoxManage convertfromraw --format VDI "source file path" "target file path" ).
In the settings of the VM, you have to use as storage manager: SATA (override all others). In first pausition put the IMG file converted to VDI, then all other virtual disks.
In the system settings of the machine (tab motherboard), choose the number of processor and memory allocated (2 processors for me and 2048MB of RAM) and especially used chipset ICH9 (otherwise does not start correctly).
In the network card part, enable card, bridge access, advanced settings, choice of Intel PRO / 1000MT Desktop card. Be careful to fill in the correct mac (same as grub).
Usb: default (use usb 1.1)
Serial port: enabled (COM1 - disconnected)
When starting the machine, choose baremetal option.
Good fun