Any point changing gearbox oil?

[LiquidGnome]

After the gearbox in my last car fell to bits, I'd like to ensure that this one has a long and healthy life.

It's an M66 in my C30 D4. The car is at 46,000 miles.

Is the general consensus that this is worthwhile, or a waste of money? I intend to keep the car for a long time.

Thanks