I believe that you can use almost any decent proper weight gear lube, regular or synthetic, and as long as you change it when you should you will be fine. I drive about 10,000 miles a year and pull a trailer a little. I change every year. easy job and not much expense.
I think that synthetic oil is great stuff. I also think in most cases it is better than it has to be. The final drive on my bike is the exception. It holds such a small amount that it must get pretty darn hot. The reason is that if I change it annually (about 8000 miles) it is pretty dark. Not so with synthetic. It remains pretty clean. Synthetic oil must run cooler therefore not burning and extending the life of the oil. Oh, also the life of the final drive. I would suggest synthetic to all who ask me for this application.