You guys are all scared of 10W30 like it's the plague or something. Up through '93, that's exactly what the Helms manual said to put into Honda transmissions. It certainly won't ruin the gearbox, that's for sure.

On possible cause of this problem is simply that the clutch is not grabbing as quickly as it used to, which causes the grinding on that shift. Otherwise your 3rd gear synchro might be wearing out.