Drained 6.5 qts of Castrol GTX 5W-30 out of my '85 Corvette track car. The dipstick was smelling "fuelish" due to a lot of mapping experimentation on the ECU. Lots of starting and idling trials, some chassis dyno time, a few trips back and forth to work, and 4 track days. Probably not much more than 1500 miles, total. The oil looked pretty thin and black for no more miles than that.
Refilled with Mobil 1 5W-30, which makes it common with my Camaro, so I don't have to keep two different kinds of oil around. Still have 2.5 quarts of GTX, so I guess that goes in the lawnmower, little by little.