5th gear is usually the first to fail as it is furthest away from the oil supply. Chances are if you were to drain the oil you will find lots of metal in there

Those bits of metal will also be flying around and taking chunks out of the rest of the cogs in the gearbox. Personally I'd drop the oil from the gearbox and see what comes out before venturing too far with it. If you're lucky, I'm wrong
