The gearbox doesn't have 100,000 life expectation? Not sure where you've heard that? The cars, when new came with 4 years or 100,000 miles warranty? But everything can still work way past that.
Your car is out of warranty anyway, so you don't need to deal with Renault, a 197 gearbox you can pick up for around £300-400, or get yours reconditioned for similar money, that said, if yours is feeling slick then I wouldn't worry too much.
Just use the genuine oils, they're more than up to the job