Over the years, I have become convinced that synthetics are more important to use in transmissions than even in engines. Many of us, at one time or another, have had transmission problems, which caused us to embrace the added protection synthetics offer. I started using them in my engine and transmission in 1978. I started using Eon, which became Ultron, which was acquired by Exxon and, as you know, marketed in Europe.
Having tried many synthetic gear oils and ATFs over the years, I believe Amsoil Synthetic ATF is the best. Next in line is Mobil 1 ATF. However, I need Mercon V for a 2000 Taurus, with a high performance Duratec engine and AX4N transaxle combo. Dyno hp is greater, shifting is smoother, more precise, and fuel economy is better than the Pennzoil Mercon V I was using. Similarly, since I live in Texas, the transmission runs cooler with Amsoil than it did with Pennzoil. A friend of mine, who used to haul steel in his Ford F-250 diesel with an automatic transmission all over Texas, told me about Amsoil 7 years ago. He had used Mobil 1 ATF, but preferred Amsoil from his experience. Many BITOGers love to bash Amsoil, particularly G-Man II, but I've had nothing but great results with many of their products. Having been to engineering school and having worked on cars, trucks, and motorcycles all my life, I feel my advice is better than most. In my Ford's automatic transmission and power steering I use Amsoil ATF. In my 1993 Civic, with a manual 5-speed, I use Valvoline SynPower 10W-30. I have used Mobil 1 10W-30, but the Valvoline product is smoother shifting and quieter. Since you mentioned transmission oil, I was not really sure if you were interested in ATF, gear oil, or other. Hope this helps.