Yes, frictional properties are also important expect the 5R110 doesn't have any REAL need for FM different from MercV. The reason why the 5R110 was spec'd for SP is because everything during that era (except for CVTs) went to SP for fuel economy reasons. The same oil is in cars, vans, etc.
As I said, I run MercV in the fleet trucks. Combined mileage on the 5R110 equipped units is well over 2 million miles with no fluid related failures, so I feel comfortable running that fluid in them.
Some transmissions are much less sensitive to FM formulation changes. The SuperDuty is fairly immune. Remember when the E4OD's were new in the late 80's through 97 and then it's updated version 4R100 into 03 and Ford said ABSOLUTELY no use of MercV? They denied warranty claims over people incorrectly using MercV in the E4OD/4R100 transmissions. Then a few years ago they said well we aren't licensing Merc anymore, ok it's fine to use MercV in those transmissions. And there were few failures and those that did usually had other contributing issues.
The 5R110 is no exception. Ford would like everyone to buy SP as it's more expensive than other ATFs they sell and in applications other than the SuperDuty's the SP spec'd fluid is much more of an issue.
I'm not saying you should run any other fluid over the SP, but I am saying you can do so and not hurt anything providing the transmission is in good shape.