Not my truck but the guy helping me do my roof has a 91 S-10 Blazer 4.3 auto uses a quart of oil every 150-300 miles. No visible smoke when following him, nor on startup, just a slight leak at the drain plug and filter extension adapter but doesnt seem like 300 miles/qt worth. Any help on where to look, or could this be burning oil without clouds of blue smoke? The truck has 150k on it. He has been using 10-40 lately with no improvement in oil mileage. Also tried a can of motor medic(?) no difference.