Currently the recommended thing is that for the inventory adjustment we make an average of the real stock and in that way we load it to odoo, in case of having to make the adjustment and we want to have the real values (With FIFO valuation), we can do it from an inventory receipt (To be able to do it we need the Studio app).

First we must activate a field in the stock.move model view.

Studio > Inventory App > We enter a receipt >

 From the reception lines (Stock.move) 

From the "existing fields" section, add the "unit_price" field to the view.

After having enabled this field in the receiving view, we must generate the inventory receipt with the actual order of the products as they arrived so that Odoo takes the FIFO inventory values. 

Example:

We have in one reception, the same product in 3 different lines. These have a different price and a different quantity of reception.

According to this information, the incoming inventory value should be as follows:

$10 x 10u = $100

$15 x 20u = $300

$30 x 15u =$450

Total: $850

Once we validate the order:

Here is the valuation layer:

When we make a delivery of this product

20 unit sale

You should take the first 10 at a value of $10 and the next 10 at a value of $15, this would be an inventory movement of $250. 

And in our inventory would be a total value of $600.

Same process to products tracking by serial number or lots.