It has different additives but in reality unless you are providing strenuous activity on the axle day in and day out. You will never notice a difference.
I don't think that's a fair statement. There are still a lot of benefits that even a casual drivers can appreciate. Lower temperatures and dramatically increased lubricity are going to reduce wear on metal parts and prolong the life of seals. Even though it's twice as much in relative terms, we're only talking about the difference of $100 or so. If that extra lubrication extends or prevents the need to replace a single part or seal, it has paid for itself easily. Factor in that we don't have the Army's logistics chain dropping off "free" parts, and it's even more valuable. Unlike the Army, who has to make cost decisions based on outfitting 100,000 of these trucks, we can easily put higher quality fluids and components into our one truck, to take care of it as best as possible. The synthetic oils' abilities to resist thermal breakdown and oxidation also make it last much longer, meaning you should be able to extend your oil change intervals both by time and distance - that itself offsets the cost.
The lubrication benefits of synthetic oils, especially high-end types, are pretty amazing. There are lots of scientific studies on it, but I still think back to an anecdotal story that is easier to visualize. When you get conventional oil on your hand, you can usually wipe it off with a clean paper towel to the point where you can easily grip things again (e.g. grip tight enough to open a tightly closed container). With the high-quality synthetic oils that's not really possible - even if you use a couple of clean paper towels, the tiniest amount of residual oil still makes it extremely difficult to grip items securely. It's a clear difference, you can literally feel. There are also numerous emperical tests you can watch in YouTube videos (yes, some are more relevant tests than others).
There's really no downside, other than the extra $100.
I'll look into that Spicer gear oil. Unfortunately, it probably won't be as easy to find tests/studies/shootouts between commercial truck products, as you can for consumer products. I'm not sure Spicer's oil being the OE for a bunch of brands really means it's better oil, because again they are likely to need to balance the cost of supplying a great number of trucks. I'm interested in whichever performs the best, if they're all in the same magnitude of cost. (I'm not saying that Royal Purple does perform the best against Spicer - there's no evidence either way - but their other products show a trend that their oils perform extremely well across the board. So I think it's a good recommendation, too.)
Similar to Suprman's experience, after I put the Royal Purple (synthetic) oil in my front hubs, there was a drastic decrease in temperature after long drives in the 95°F summer heat. The hubs now run about 20°F lower than before, and are barely warm to the touch after a drive (e.g. previously around 125°F, and now around 105°F). Yes, I see your argument that the temperature change doesn't itself gain me anything, but that means that there is significantly less friction on everything involved, and that means less wear and tear. Just to fill the hubs is about a $5-10 difference.
