1988 Toyota Corolla Transmission takes long to shift into o

I have a 1988 Toyota Corolla with very low miles (114000) and has been very well kept. It has a 4spd overdrive automatic transmission. When I drive the car cold it takes forever for it to shift into overdrive (I can hit 70-75mph on the highway). Eventually it will warm up and seems to do better. All the other gears seem to shift fine. I had a trans filter change (thinking that might have been the issue) and fresh trans fluid/Lucas transmission oil and it still does it. Thanks in advance for any help!