Take the car to a trusted mercedes benz mechanic. make sure they have experience with 560SELs. leave the car with them overnight. ask them to have another tech start the car COLD while they listen to the engine for unusual sounds upon start up. if they hear something, then further diagnosis is required, if not, you can begin saving up for the repair.

also, you will want to use a motor oil that has zinc additives due to the valves/hydrocompensators. Chevron Delo 400 or Amsoil Premium Synthetic are good choices.