HF: Oil of Fortitude - How/when does the game determine the increase value?

[Kunibert]

Right after taking a bottle of Oil of Fortitude from Gillian's stash, using the oil gives a random durability increase x on the first try. (Reload your state, take the oil again from the stash and use it on the same item: the increase should be different than before.) ...well, not always. Some times it seems to keep static. So far i couldn't find out when it is random and when it is static, but it seems that saving the game (of course before using the oil) sometimes changes the increase also.

But if you save the game with the oil on the floor or in your inventory, the increase y after every reload is exactly the same.

Also, entering another dungeon level changes the increase to z. Reloading and entering the dungeon again: The Increase is the same as before. But reloading, entering dungeon, saving and reloading: the increase u is different than before, but still static. Changing between the same two dungeon levels won't help (e.g. Town and D1), but Town -> D1 -> Town -> D5 -> Town -> D1 should change the increase you had the first time in D1. (I am not sure about that.)

So: How/when does the game determine the increase value?

I ask since I am about to oiling (blacksmith) the max durabilities of Unique Items to 95, then using Oil of Fortitude (on two handed weapons only from now on) up to 145, 195, 245 and finally use a hidden shrine to make the item indestructible. (One handed weapons are shrined after oiling up to 95.)
I was able to use Fortitude Oils twice to gain the desired increase so far:

The first time on The Gladiator's Bane which i shrined up before from 135 to 195, oiled (+50) to 245 and shrined to 255 which is the exact value you need to hit to make the item indestructible. (I did not know this at that time, so I was just lucky.)

The second time, when I had my Veil of Steel shrined up to 140 max durability already, I learned that 255 has to be hit in order to reach indestructibility. I was lucky to use the Fortitude Oil to hit 185 max durability.

Now I want to use the third Fortitude Oil i found so far in 16 times i played through the game. It is pretty rare, so I really need to hit the +50 increase.

(BTW does the Oil of Permanence ever drop? I did not find any of these so far but have a base AC 4 Thinking Cap waiting for it...)


regards

Kunibert

[Kunibert]

After some further testing I eventually reached the desired increment. Here is how I did:

1: Have your inventory opened, containing the Oil of Fortitude and the item whose durability you want to increase.
2: Save your game.
3: Kill an enemy. Do not save your game, but use the oil: The "1st try value" of increment seems* to be random. If the desired increment isn't hit, reload and retry this step.

*I luckily reached the +50 increase on D15 after three or four tries whilst testing and decided to save my [Dur 145] Windforce, so I cannot tell if the increment really is random after killing the same enemy over and over again.

But there also are some other things you can do to change the "1st try value":

Right after opening a chest or destroying a barrel, a different increase is hit on the first try before reloading, but always the same value for the same chest/barrel.
It also seems that recharging a staff or selling an item to Griswold or Adria changes the "1st try value", but I could not test enough to be sure about it.