There are only a couple of things that will kill a 300E. I would start by looking at the wire that connects to the EZL...If a mercedes shop looked at it, they may have taken off this wire to prime the oil. If not, I would look at the fuel pump relay. You can pry the bottom open and look into it. If there are burned traces or spots, you have found the problem.
ASE CERTIFIED MASTER AUTO TECHNICIAN
MERCEDES SPECIALIST 11 YRS