Be careful with synthetic, unless it specifically states that it's appropriate for use in manual transmissions. Some synthetic lubes can be too slippery. The synchronizers need a certain amount of "grip" or you'll have trouble shifting.
What does your manual recommend?
Every Renault delivered is filled with synthetic, both in tranny and engine, so I don't see why you should be careful with it. This is a myth that occured in the beginning of the seventhies when Mobil1 had an insufficient add-pack with their new synthetic. Kestas grandfather told him to be careful with synthetic oil, and now he is trying to tell you the same fairytale. He just doesn't know we are not stuck in the seventhies. We have come to the year 2006!! Use synthetic oil and your engine would last longer, as well as a slightly lower fuelconsumption and longer drain intervalls will make up for the higher cost.
Jonny, I didn't pull my advice from the air or from my grandfather. Many here on the board have reported problems when using synthetic in manual transmissions without checking whether their choice of lube was appropriate. When inappropriate, shifting can get horribly notchy. I have nothing against synthetic fluids, per se. I'm just saying be careful and know what you're doing. So back off and do your homework before getting personal.
I don't think it matters whether you use synthetic or conventional gear oil as long as it meets the appropriate specs and is suitable for your climate. There may be some brands that don't agree with your tranny though.
What info does the owner's manual provide on gear oil for this car?
Many have reported substandard results with Mobil One synhetic gear oil, when used in manual transmissions. It doesn't shift right, becuse it is not a gearbox oil, but a differential oil.
The best gearbox oils are synthetic oils, but they have to be designed for being in a manual transmission.
I don't know what is available in Venezuela in your area, so it's hard to recommend a specific name.
Using the fluid that is recommended by the Manufacturer would be safe, and should be readily available.
I know little about transmission oils, but would nonetheless suggest a change every 30,000-40,000 miles, or two years.
As to the question of dino vs. synthetic, all I can say is from my experience, which is that I put Amsoil's FGR, a 75w-90 GL-5 gear lube that is MT-1 rated, in a '93 Toyota, for which the manual spec's any of GL-3, GL-4, or GL-5, and it shifts perfectly. A different oil may be better; I don't know. But I'm happy with what I have.