Transmission problems. Expert needed. Grand Prix.

Hi! Is there a transmission expert available. To answer my main question please? I have a 1990 pontiac grand prix ste turbo. It has a turbocharged 3.1 liter v-6. It has an automatic transmission with these gears. Park, reverse, neutral, drive, 2nd, 1st. It is a front wheel drive car. About 5 years ago, the transmission started giving me trouble. Oneday when I went to put the shifter in drive, the car would not propell forward. It acted like it was still in neutral even though I had the shifter in the drive spot. I put the shifter in 2nd and in 1st, same thing it acted like it was in neutral and would freely roll but would not propell forward. However the car woild go in reverse just fine and neutral worked and park worked also. So, after I repeatedly kept shifting the gear stick back and forth several times, finally after repeated tries the drive gear started working again and the car propelled forward. When it was not working it felt like it was not engaging properly and like it was still engaged in neutral even when I had it in drive. Ok, after I finally had it working properly, oneday I went to crank it and put the shifter in drive and it stopped working again and it would not engage or propell forward. Same issue with 2nd and 1st gear, would not engage, felt like it was still in neutral. Yet, reverse, park and neutral worked just fine. Again, I repeatedly kept moving the shifter back and forth through the gear spots but this time I could not get the car to go forward again even after repeated tries several times. After four years went by I always tried periodically and oneday I put the shifter in drive and it felt like it engaged and sure enough the car goes forward again. I have done nothing to it. Why does it periodically correct itsself? What is causing this sparatic problem? And why does the issue only effect the forward gears anf never reverse or park? The transmission fluid is always full level. Will it most likely stop working again and if so how can I fix it if it does? Please help me with advice! Also, next time I have an oil change for the engine, can I use fully synthetic oil like mobil 1? Its an older engine and I have always used regular oil and did not know if synthetic would hurt it? The tailpipes smoke alot but I can find no oil leaks