I can't provide any scientific proof just real world experience aftert 40+ yrs of driving. When younger, I drove some real high mileage beaters and never touched both the transmission nor the rear differential with no failures that I can remember. I currently own an award winning '82 Volvo 240DL (all original, original owner) with over 230,000 km. Again, have never replaced the tranny fluid nor the differential, however I did replace the clutch a couple of years ago, only because it was starting to show much wear, and while replacing the rear engine seal, it made sense to replace the clutch at the same time. Also, I recommend not touching the tranny fluid (assuming their are no issues) as again, I have witnessed first hand, perfectly running cars with auto transmissions, that had their owners replace the tranny fluid (automatic transmission) because they felt it was the right thing to, only to cause days/weeks later, catastrophic transmission failures....my understanding is that most differentials are factory sealed with no access to either check or drain the fluid....just my two cents!