How often to change rear end oil.

If anyone decides to change you're rear end oil here's a tip that believe it or not is a fact use synthetic gear lube ( I used Royal Purple Brand ) as it does make a huge differance in the gear life, I had an 8-3/4" rear in my race car before I put the Dana 60 in and used a log book and I got 50 runs out of a set of gears as my car left real hard with the Royal Purple oil I got 75 runs out of a set ??? If someone told me that I'd have call it bull **** but I can tell you it is true . I sold the car years ago and the Dana 60 has never broken a gear set so if you have some serous HP use a Dana 60 and Royal Purple is the only way to go .