I ran the "never replaced just top-off" gear lube in my 2001 Suburban for 90K miles and it came out looking pretty darn good. I got swayed by the owner's manual warning that no other fluid should be used so I mixed 1 quart of the $29 a qt GM stuff and 1.75 qts of the Amsoil 75W-90. It's been almost 30K now and no problems.
The front diff OTOH, with it's conventional fluid 80W-90 and required service intervals, looked really, really nasty when I took it out at 90K (unsure of previous maintenance history on the front diff). The front diff fluid was gray and had a LOT of metal in it. I used Amsoil 80W-90 and replaced it again at about the 110-115K mark. While it wasn't gray or nasty looking, it did have metal particles in it (nothing like the first time though). Hopefully now it has been flushed out and when I change it at 150K it will be clean. Goes to show the difference between synthetic and conventional. The front diff rarely has power through it (use 4WD maybe twice a year) yet looked horrible. The rear diff always has power through it, is never recommended to be changed, yet looked pretty good. The GM stuff has proven to me to be very good but is sooo expensive. If someone can reassure me that it does not need to be used at all, then I will go 100% with a $10/qt fluid. Also, I'm thinking about going with a 75W-90 in the front diff. I don't see why it needs an 80W-90.