Hi everyone, I had that exact same 3rd-gear crunch on my old manual truck a couple of years back. Switching to a high-quality synthetic fluid helped reduce the harsh noise for a few months, but it was ultimately just a temporary bandage.
Once those internal synchronizers start wearing down, you are almost certainly going to need a proper gearbox repair.
My advice is to tackle the issue early before a failed gear damages other internal components and turns a minor overhaul into a much more expensive gearbox repair.
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