Genuine OEM Brother MFC-8420 Toners
The MFC-8420 uses TN-530 black toner. TN-560 is its high-yield choice. DR500 is the separate drum, so the cartridge capacity decision is independent of drum maintenance.
At Toner Buzz, genuine Brother choices cover TN-530, TN-560, and the DR500 drum. Select the toner cartridge that fits your printing volume, and use the drum part only when the printer indicates that DR500 needs attention.