We must not forget that due to the age of these cars that most of these cars have had multiple owners and the O-ring probably has been replaced before. Also, leaks are quite visible due to oil on the ground and or spread around the underside of the chassis.
Depending on the mileage of the car and the state of the oil should determine if an oil change is required.
Most limp modes have been caused by the electrical/solenoid plate.

Save your money, and before you do anything else get any codes read and check the transmission oil level, this is a pain as there is no dip stick for it, get one or make one as described in this forum.