3-speed manual leaking fluid

Thanks. I have another question about the 90w GL-4 gear oil. About 25 years ago, I had some work done on the transmission and, when I got it back, it felt like there was molasses in the transmission - I could barely get it to shift on cold mornings. I took it back to the shop (AAMCO) and the owner said that it needed automatic transmission fluid rather than the 90W that he'd put in. He changed it and it has worked fine since. I never double checked this info until now - so your answer makes me wonder why he used automatic transmission fluid, why it gave me trouble when he used the 90W, and why my transmission hasn't had issues with the wrong fluid in it all these years.