Classic problem. On most gearboxes of that generation from everday manufacturers, 5th is the first to show signs of
starvation as it is still essentially an extra unit bodged on the end of a 4-speed (newer 5 and all 6-speeds have an all-new 3-shaft design). The need to preserve external clearance and the smaller output gear compared to 1~4 means the extended part has a shallower sump. Starts off being noisy as it's short on
, then eventually it runs out entirely, the bearing overheats and breaks, and you lose the gear. If you're terribly unlucky, you can lose the whole box (metal shards whirling about). 5th is at least more easily replaced than the other gears, but the labour costs/trickiness of the job and the need to remove it anyway means you're as well to just replace the whole thing.
I would try to avoid engaging 5th or racing the engine too much for now (taking the load off the bearings will help but it'll still be spinning along with all the others), and get hold of some top-up
as soon as practical. If I remember rightly, 75w90 gear
should do the job, and the filler/dipstick arrangement is just a plastic jobbie that can be turned by hand, at least after the seal formed by years of direct exposure to road crud is broken. Might still be hard to access though - I had to pour the
(pre-warmed by floating the bottle in a bucket of hot water) down a length of cut-off hosepipe last time I had reason to top up a gearbox.
Also, before anything else, try to find out if there's a reason why it may be low. Is the casing cracked and leaking anywhere? 5th gear unit cover gasket perforated? Most gearboxes are the epitome of "sealed for life" and topping up more than a couple hundred millilitres over 100,000 miles should be viewed with suspicion. The times I've done it were mainly after changing the transmission (going from a 4 to a 5, or replacing an abused one) as transporting them and potentially having to take flanges off the diff means pre-draining is a good idea.
Oh yeah and be gentle with 2nd, at least for now... the synchronisers depend on the box
to work properly, and I'm living proof that Renault boxes have fragile 2nd gear synchros.