So i have noticed that the salve amulet takes less priority than a manta ray when you try and equip yourself for a wildy trip to spectrals and you bring 3 items slayerhelm, chaotic maul and salve amulet + full food and prayer potions. is it possible to increase its value which decides what you keep so you can keep it on death and not lose everytime you die?

You will keep the most expensive items. However, due to the items value being based on their high alch value, the untradeable items don't get prioritized as expensive items because simply untradeables don't have any high alching values. That's why the manta ray is protected whilst the salve amulet is not.