The seal restorers you buy over the shelf work by swelling the rubber in the seal. It does this with an additive ( many times some type of alcohol). This additive is aggressive to the rubber, and swells it like a dry sponge getting wet. The problem with this is you loose the durometer ( hardness) of the rubber, and wear can progress exponentially. The effect on the seal is dependent on the rubber formulation of the seal. So you might have less of a leak due to the swelling, but its possible it wont last long, and wear quickly.
These trucks last so long, leaks on rubber rotational seals are bound to happen long before the engine dies.
A bit of history: in the late 80s/early 90s many seal manufacturers went through a process of reformulating their rubbers to withstand synthetic oils. Mobil 1 was particularly aggressive in those days. It is possible the seals in your truck are not formulated to withstand the synthetic oil. Also, synthetic oil does a good job of finding a path to leak where a normal dino oil will not. There is no ill effect on the metal parts from the synthetic oil, but the vintage of our trucks may not have the seals to withstand it.
The only way to stop a leak On a rotational or reciprocating seal is to replace it. If there is a groove in the shaft where the seal rides, change the installation depth of the seal so the lip will ride on a new surface. Be sure to install the seal square in the bore. Be careful banging the seal in, you dont want to bent the metal case in the seal, and you do not want the seal lip spring to jump off, it can do that with repeated hard banging.
Just so you know Im not full of hot air, I have 15years engineering experience in powertrain sealing with an OEM supplier. One of our largest customers was Ford, although I did not work on any of these trucks.
Good luck with the fix.
