Syns are most likely all about the same, to a point,RP being the most expensive than others, which in no way makes it any better perhaps.
Have been using RP 5-30 in my engine,RP ATF in my auto tranny and Max gear RP in my diff since 5K miles, have had 0 drivetrain issues pushing 25K.
Mercon v is most likely good stuff, has been a rash of recalls on 04s to put in an additive tho.
has been lots of people that have had major problems with the 5R55E, just maybe using RP syn, the lifespan of that tranny will be better, maybe so, maybe not, cannot prove anything, it will be a long term experiment I hope.