Are you sure it's 4-stroke ? you say it has seperate oil/fuel reservoirs, my old 2-stroke motorbike had this arrangement and mixed the 2 ensuring a constant mixture, 600ml of oil sounds about right for this 2 stroke arrangement but quite low for a 4 stroke engines sump. Do you poor the oil down the dipstick tube, or is it a seperate tank ?
If it is a 2 stroke that mixes it's own oil/fuel then using 4 stroke oil will no doubt reduce the lubrication alongside a lean mixture hence the engine seizing quite quickly.
Pete