Bought a 2002 certified sts w/10,000 miles. After 18,000 miles low oil message light came on. Dipstick indicated hardy any oil present. Still 27 % oil life left. Dealer said that this was a common problem with the northstar engine and oil should be changed every 4000 miles. What's going on? Is he blowing smoke? Another issue is that mileage is only 22 mpg on the highway, based on previous 1995 northstar engine I was getting 27 highway. Are these common issues ?