The oil pumps upward through the cam bearings, lifters, push rods, and onto the rockers, the runs through the return passages to the pan. When the returns get blocked, it will usually start off fine, then at higher speed, the oil light will come on (oil going to top end faster than returning to pan, pump starves), and if you stop the motor, it will show on the dipstick as being out of oil for several minutes until the oil makes it past the restriction back to the sump. Real common in some older Ford engines, haven't relly seen it on GM (not a recent one), but it could happen.