I too have a 1966 230 SL and was similarly concerned. I talked to Bruno at Pacific Fuel Injection in South San Francisco and he advised that the oil should be measured by the screw-in dipstick at the rear of the pump (not the red plastic breather cap). The pump can supposedly be drained by a screw below the knurled adjustment knob (the one that sets idle fuel flow), but I haven't done this yet. I did top up the oil level when I started my restoration as it was nearly empty.