My first thought is old or cracked pan gasket, tightened bolts no help, crazy that you cannot remove the pan without removing the motor/tran, what were they thinking, is there any other cure out there for this issue, seal softer additive maybe? motor has 171,000 miles on it. still runs ok.
Oil driping from oil pan on the passinger side from the four rear pan bolts on 1999 Cadillac DeVille
Very common problem on that engine. Head gaskets will be next.