2000 Corolla with 108,000 miles on it. This car is using about 1 qt of oil every 1000-1200 miles, which to me seems a bit high. It does not appear to be burning oil and I cannot find any leaks. I've been using 5-30W oil as recommended but the last change went with 10-30W to see if that would make a difference, but the consumption rate seems unchanged. I think consumption is increased when running at interstate speed of 70. Any ideas as to the cause?