I you believe the owners manual, 1/4 down means you have used approx. 1/2 qt. of oil.
1/2 qt. of use in 1500 miles in an M103 motor ain't too bad. I wouldn't worry about it 'til you're using a qt. every 1000-1200 miles.
Steve Brotherton has pointed out that the biggest problem is worn valve stem seals. MB switched to better valve guides in Apr. 1988 - M103. Look along the rear of the cyl. head - passenger side. If you see oil leaking there, it may be time for a new head gasket. M103 motors sometimes leak from this area. Make sure it's not the valve cover leaking if you have oil in this area.
I had valve stem seals, new head gasket and upper timing cover u-gasket replaced on an M103 motor last Apr. and my consumption went from 1 qt. every 1200 miles to about 1/2 qt. every 2500 miles.
My 2 cents.