Dave,
If the oil stayed below 212F, the oil will oxidize very slowly and, IMO, a "lifetime" 100K fill is possible, once the break in materials are flushed at 5-10K. I hedge a little on that long OCI unless the diff has a magnet... which the Mag-Hytecs do... because contamination from wear metals is an issue in a filterless system like a diff. A good magnet does an admirable job in keeping the iron low. I like the Mag-Hytec because the magnet is a dipstick and you can pull it periodically to clean off the iron without having to drain any oil.
As with ATF, IMO, occasional and short spurts to higher temps up to 250 aren't a problem. It's sustained high temps that lets oxidation rear it's ugly head and degrade the oil.
You'll find that most trucks will run about 160-180F on the freeway empty, the range accounting for ambient temps. It takes a while to reach temp (when unloaded), especially with a system that holds a lot of oil. The oil lasts forever at those temps. Beyond that, it's all about load and that's so highly variable that I can't venture a guess. That's why you need the gauge. A sustained 225F IMO is at the upper end of normal (with a good oil) and well capable (again in my opinion) of lasting 40-60K miles (depending on the oil). A high quality oil maybe longer.
Here are some things you can count on as far as diff oil temps go:
a) More load= more heat. Load could be a carried or towed load, but also windage from speed and typical poor truck aerodynamics, grades, rolling resistance from sticky tires, etc.
b) A smaller ring gear will run hotter than a larger at the same load. A greater hypoid offset will also run hotter. Kind of a moot point these days since most diffs are around 1.5-in. Some older diffs, like the Ford 9-in. had greater offsets (2.38-in.) so ran a little hotter by nature.
c) A lower ratio (numerically higher, e.g. 4.10:1)will run hotter than a higher (numerically lower, e.g. 3.07:1) ratio.
d) Oil volume only delays the inevitable temperature but more volume does account for better cooling because of more surface area for cooling and a little more dwell time for the oil.
e) A slippery, friction reducing oil can account for a drop in temp (vs a lesser oil... it's all relative). I found a syn oil ran 10 degrees cooler than a "ordinary" conventional but I also found a well formulated conventional straight 90 that ran even cooler than the syn. I also recently saw lowered temps by going from a syn 75W140 to a conventional 80W90 and saw a small drop (5-10F) in normal operating temp.
f) Traction aids (limited slips & lockers) seem to have little or no effect on oil temp in normal driving.
g) Fluid friction from a heavier than needed viscosity causes higher temps. Ditto for overfilled units.
h) More speed= more heat. The faster you go at any load, the higher the temp.I drive around at 40 mph, the diff barely crack 100F. Drive 70, it runs at 180 in the same ambient temp. A good part of that is load, of course. I fairly regularly tow up to 31 tons (that's TONS, 62,000#) of grain wagons during harvest (braked trailers) to the elevator at the legal farm SMV 25 mph and the diff temp barely reached 150F after 5 miles on level ground. Towing 9K pounds at 60 mph takes me to 225F on level ground.
Some years back I did some testing and you can read about it here:
Diff Temp Test
