Had my Hyper 7 PBS for 2 years, bashed and raced it heaps and never once changed the diff oil When it got time to sell I stripped it all apart, the diff gears still looked 100% and the oil still felt good.
Haven't yet bothered to change the oil on my Hyper 8 either and thats had around 10-15 litres through it with 2 different engines, not a problem. I change the shock oil every few months, but a good quality diff oil should last for ages, unless you got a Matrix that leaks