Yes, I'd understand your reluctance to run your oil at a lower level to stop the leak instead of replacing the seals which are labor intensive and can be expensive. Replacing the seal is the conservative thing to do if an oil swap doesn't solve the leak.

However, I've actually been doing exactly this. No my diff hasn't blown up or making some awful noise. I have a leaking front differential at the driveshaft seal. It was discovered after I did a front diff service, and I had my indy take a look at it. We opted to lower the diff fluid to near the minimum amount that BMW specifies in their service manuals and it solved my leak inexpensively (don't remember the minimum recommendation). Its been running this way for 5k miles without any obvious issues (no leaks and no change in performance). Diff fluid like all fluids (oil, tranny, etc.) in a car have an acceptable range. My indy has had success with this approach of lowering the fluid level to eliminate leaks, assuming its not leaking at the case seal or the drain bolts.

While diff fluid provides some of the cooling, there is an acceptable range of fluid levels needed to provide sufficient cooling. We already know since he's leaking and I'm at a lower level and we haven't blown our diffs so long as your above BMWs design minimum you'll be fine. Adding dry lubricant like NoS2 could help as well.

Something else to consider, I see diffs for this car going from salvage yards in the $300-350 range for low mileage grade A examples, which means diff replacement isn't horribly expensive if running fluid at a low level leads to "premature" diff failure.



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So why do I gamble a little? 1st this isn't my only car. 2nd I purchased my X5 used its a 2008 4.8i fully loaded and currently has 92K miles and is worth about 10K on trade in. In the few months I've owned it I've spent ~5K in maintenance and repairs (new alternator, new CCV system, all fluids changed/flushed, new rain tray, new spark plugs, air filters, front suspension refresh, new coolant sensor, new maf). Its very easy to spend 50% of the cars worth in repairs/maintenance per year. Which makes you question why even keep an old car if its the same cost in repairs and maintenance as you would pay in depreciation on leasing a new X5. So that's why I do my own work where ever possible and as much preventative maintenance as possible and watch minor issues. I'm not tied up in the fact that this car was over 70K new, I just look at it as whats it worth today, and does it make sense to spend money right away on minor issues when there are plenty of other things that will break and be expensive to fix when the vehicle is currently not worth a whole lot.