This.

My sifu mechanic last time say drain out abit, and pour in new, sort of slowly 'dilute' old oil concentration

Worry if fresh oil will unclog all deposits and lodge it in the components, causing more harm then good

Maybe darin out 25 percent, then pour in new, after few k km repeat, then after few round no issue then replace.

But in any case 260k km, the gearbox has served you well