i have a 1991 oldsmobile delta 88 did this same thing after 209.339 miles 3.8 v6 4t60 automatic tranmission 0 oil usage drive 56 miles round trip every day, great power tran.i have a few clues that might help maybe.the problem was out side the transmission.one i had a loose park neutal switch. when you come to a stop engine and trans shifts forward wiring harness fasten on one end near the frame pulls against the switch causing the switch to loosen up after a few dozen years could not beleive my self till i saw it my self it puts tranny in neutral or park electrically, all the electrics is wired through this switch beware. number two problem i found was vaccum line to modulator valve was near throttle positioning contol was collasping closeing off vaccum.third problem i found was the tranny cooling lines under radiator were leaking causing oil pressure loss on torque converter causing convertor to unlock.this transmission is noted for torque converter failure.this car has a 4t60 it has a tv cable and vaccum modulator, electric speed control sensor on tail shaft passenger side of car.4t60-e is all electronic no tv cable no vaccume modulator 4t60 1989-1991 late 1991 till know all 4t60-e fully electric controlled no machanicals no vaccums. try trans-x transmission conditioner add one third of bottle over three or 4 days stop and add another bottle again over same amount of days work in slowly when hot also add trans sealerstop leak one bottle add all at once do not drain out any fluid will not hurt tranny. i beleive you have a 4t60-e last month 1991 and from 1992 on all olds.buick.chevy uses 4t60-e. varnish has built up and valve control system is sticking oil treatment and sealer will polish surfaces in valve system and unstick them. i will close know hope this helps i am honored to help if i can. 4 (means 4 speed) t (means transaxle) 60 (means) toque converter for mid engine turbine pitch and amount of blades it has. 3.1 3.3 3.8 etc if it was 80 this is for v8s and desiel engine 4t40 is small engines 4 cyclinder and smaller. have a good day.