Based on the "Magic Items Awarded by Rarity" table on page 135 of Xanathar's Guide to Everything, we should expect a group to acquire 1 minor item of the Very Rare category somewhere between levels 5 and 10. Far from being too early, this is about the right time for the party to start bumping into the occasional Very Rare consumable item. There's no need to downgrade its power level to make it fit.
Xanathar's makes explicit what was merely implied in the DMG, that there's a distinction between major and minor magic items. Minor items are on the Magic Item Tables A through E in the DMG, and the rest are major items. In general, the minor items are the consumables (including potions, scrolls, and ammunition) plus the items that you could consider 'magical adventuring gear' (as opposed to armor or weapons) -- items such as bags of holding or horseshoes of speed.
In any case, even a powerful one-use item won't break your game. The worst it can do is to potentially trivialize, to some extent, a single boss battle that you had meant to be scary. And frankly, if your party uses their one shot of a powerful effect to win a scary fight, then the game is working as intended and you shouldn't feel bad about that.
On the contrary -- cheer on the fighter (or barbarian or whatever) who's blowing a big important resource for this battle. Play up the fact that they're wrecking house against an enemy that's stronger than they are. That's a big, fun moment!
Don't worry, nobody is going to walk out of that feeling like they were handed a victory they didn't deserve. Quite the opposite; the players will feel like they won because they were smart and used their secret weapon at the right moment.
Now, I would definitely recommend that you tune your final boss battle to play into that specific item. Not that the fight should be impossible without it, but I would make sure there's at least one big tough monster to fight that has pretty good AC and lots of hit points, or possibly resistance to nonmagical physical damage -- something where it will feel good to come in with the oil's bonus and really hit hard. If the fighter uses an irreplaceable oil of sharpness and then spends the whole fight running around trying to deal with a wizard that keeps teleporting around and using invisibility and mirror image but goes down with just a couple of solid hits, it's going to feel like a waste of the item.
