According to wiki the difference in durability is Starmetal Pickaxe=500, Eldarium Pickaxe=375, this is without a thrall though, but still it likely won’t add more than some seconds to the durability, so while Starmetal does have higher durability the harvesting power is less 
Considering the rarity of some resources it could be worth using the Oils, personally I don’t think so since loot now is so plentiful, I mean there are crafting resources that used to require a lot of work to obtain, but with the new loot system I find myself getting plenty of all high tier resources, so much that there are several of them that I haven’t actually harvested or crafted them myself.
Things like steel, hardened steel, various lotus, gold, silver, silk, etc. is so easy to get enough of just from loot and dismantling of various drops that you don’t actually need to go find them yourself.
I find myself pretty much just harvesting rocks, wood, bark, fiber, hide and a few other basic ressources, so that was pretty much my reason for asking if Oils are useless!
We started on a Siptah official pve server 4-5 weeks ago and haven’t had the need to harvest and craft the majority of high tier resources at all, instead we have chests overflowing with high tier resources from loot and dismantled items, way more than we probably ever will need already at this point and we have begun to even throw away some of them, I mean who needs thousands of hardened steel bars, lotus and other rare resources that you mostly only need in small amounts - They have sort of turned the game into a looting game instead of actually having to go out searching for a lot of stuff 
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