Browse SAE 90 gearbox oils for manual transmissions here. If you're reasonably handy, replacing your vehicle's gearbox oil is usually a straightforward task... in fact, replacing gear oil is normally a very similar process to changing engine oil
Running a high quality gear fluid not only ensures smooth gear change but is vital for the protection of internal components and gear faces / teeth. Even if your vehicle manufacture states it's fill for life, you'll quite often find it still has a drain and replacement timescale... so well worth checking.
If you're in any doubt if an SAE 90 (or 90w) gear oil is correct for your car then you'll normally find the correct spec stated in your owners hand book. If you're still unsure, or you have a unique / modified vehicle, feel free to ask the Opie experts for gear oil advice.