According to the Lotus Workshop Manual the gearbox oil is a product which you'd never have to replace.

If only!
Any oil, gearbox or engine will degrade over time so will the gearbox oil.

We recommend to change gearbox oil every three years for road cars and every two years for trackday cars.

We've had customers complaining about a noisy gearbox. After our recommendation to start changing gearbox oil they returned and told is it made a night and day difference!