I have been doing Pokemon trades since trading is activated in game and have been in luck for some time. However, there is no exact info how does it goes since it was a “trade secret”. 
I just come to think to document my lucky trade and here it is, these are just for recording purposes and it might help or not in any way. Will update from time to time.
Lucky Trades
Cubone -> Treeko
Growlithe -> Shroomish
Cyndaquil -> Ponyta
This happens two in a row
Castform -> Luvdisc
Koffing -> Spheal
Chance is the same for every Pokemon
Logically yes… but after some time, I have the feeling that trading Kanto Pokemons to those from other regions have a higher chance than trading pokemons which are both from Kanto. Of course, except for the guaranteed old ones.
Who knows? Until the “trade secrets” are decoded, only then will it be truly known. 
Beldum for beldum (twice)
Alakazam for lapras
Rhyhorn for snorlax
Tyranitar for legacy gengar
Alolan persian for onix
Nice trades!
and how’s the rate?
5 of the 7 were auto lucky…the others are about 1 in 18ish
I don’t have either of your guy’s friend codes 
Me?
Wanna be friends ill send my code
I’ll already have your code… send me your Let’s Go Switch instead. 
By the way, have you gotten your Meltan already?
Stay on topic and pm me that
Update:
2 beldum
Gengar
Delibird
Gyarados
Blissey
Snorlax
Onix
Roserade
Golem
Sneasel
Magneton
lucky Snorlax
Lucky Persian
Lucky Aerodactyl
Lucky Espeon (Other end was a Perfect Lucky Chikorita)
Lucky Hypno
Lucky Charmander (Other end was lucky Mewtwo, had extra, only wanted one with Focus Blas)
Lucky Pikachu
Lucky Kingdra
Lucky Golduck
Lucky Kingler
Lucky Rhyperior
Lucky Absol (98%)
Lucky Mismagius
Lucky Salamence (12/12/12)
Lucky Houndoom
Lucky Grotle (98% received as Turtwig, other end was Lucky 98% Vaporeon)
Lucky Dunsparce
Lucky Nidoran Male
Lucky Lugia (Lucky Friend Trade)
Lucky Seviper (Other end got a Lucky Mawile!)
Lucky Wigglytuff
Lucky Legacy Santa Hat Pikachu from 2016 (Traded a 3k Dragonite for that one)
My first lucky-friend trade was a Shinx for a Volbeat, with someone who traveled regularly to Taiwan and the Philippines. Nothing particularly useful for either of us, but nice additions to the novelty-pokemon collection. For her something she hadn’t bothered to raid more than a couple of times, and for me a dex entry I had expected never to notch. And both turned out to have IV over 90 after the trade, which was a nice bonus.
Update:
2 beldum
Gengar
Delibird
Gyarados
Blissey
Snorlax
Onix
Roserade
Golem
Sneasel
Magneton
Abra
Eevee(96%)
Eevee
Shiny gengar for shiny gyarados, got really lucky as both were from late 2018
