honda EL63 ride on mower?

can any one tell me what the screw cap on the left hand side just above the ignition key on engine is for (as this is bone dry no oil or anything in it) I know oil is on the right hand side and petrol at back. I bought this mower with no instructions and am having trouble getting the gear in to make blades drop when you try to engage the mower just cuts out. also if any one knows where i can get instruction booklet from many thanks peter