Most line-haul truck manufacturers will suggest synthetic lube now-a-days. Most fleets use syn lube as well. Though we're talking about much larger trucks, the principle is the same with gearing. They also see much more abuse and many more miles than most of our trucks ever will.
Typically with syn lube you're looking at higher range of temp tolerances, resistance to throw off, and better boundary lubrication (film between gears). Theres more advantages as well.
When you see wear on a diff, it could be abnormal or it could be just worn out.
Surface or case hardening of gear teeth surfaces can be used to maximize wear characteristics, provide good surface stability and reduce backlash. Surface hardening is a material processing technique for increasing the (Rockwell) hardness of gear teeth surface. Surface hardening normally involves introducing carbon (at high temperature) into a low carbon steel thereby increasing the hardness for a thin material layer around the part. The purpose is to improve the wear characteristics of the gear teeth. The process for hardening is usually done after heat treat. Surface (case) hardening processes should be examined closely as there is a potential to induce hydrogen embrittlement in the material. It is important that the surface hardening process does not affect the heat treat and material properties of the base metal. Two methods for gear teeth hardening are carburization (carbon is introduced into the material surface) and nitriding (nitrogen is introduced into the material surface). Other surface hardening methods exist.
Sometimes gear teeth will be worn through the hardness and then you know they need to be replaced. Contaminated and inadequate lube will also lead to other types of wear like spalling and pitting.
How many wear induced failures have we seen on this board? Most are stress failures, shock loading etc... I think really if you just keep your oil changed at regular intervals (and extended intervals for syn) you'll be fine. However, with more extreme use of your truck, towing and severe off-roading (especially the desert guys) you're probably better off with a good synthetic.
Just my .02
edit: here are some wear patterns