Browse 75w140 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 a 75w-140 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.