Eight years ago I changed the diff oil in my 91 F250 with Valvoline Synpower 75w90. Seeing what Ford recommends, 75w140, in alot of newer trucks I wonder if I should switch? Does a thinner 75w90 Syn cause more wear than a thicker 75w140?

 

I don't know about the wear, but I like the thicker oil. I ran 75w-140 in my rear end. I highly doubt that there is that much difference though. Oil is rated by viscosity and by the degree of "protection" offered. 75W-90 is going to offer a protection of a 75 oil in winter, or cold, hence the "W" designation and conforms to a SAE 90 protection. 75W-140 is still a winter 75 and protects to SAE 140. More cushioning under heavy loads but lower ability to fill all the small crevices in the bearings quickly on side hill or cold start up.
The flip side is the 140 has more tendency to stay in the same place, like up in the pinion and so-on. A machinist explained it to me like water VS honey, the thinner oil gets where it needs to be much sooner. I bet you're fine with what you have, ford also recommends a 100,000 mile service interval on rear axles on full size trucks, seems a little high to me since randy's ring and pinion recommends 50,000 mile oil change intervals and hard working trucks to have a 15,000 mile oil service interval.