@dale in odoo we have an option of "Allow using different units of measure" in product configuration which can be set in warehouse/inventory .Its better that you create a unit of measure by referring to the below mentioned link.

https://acespritechblog.com/2012/09/24/how-uom-unit-of-measure-works-in-openerp/

or

https://www.odoo.com/documentation/user/9.0/inventory/settings/products/uom.html

then you can select the unit of measure in Bill of materials

thanks,

Old question, I know but still relevant. 

The best way I found to solve this, is to produce the "new" product with a different unit from another category.

e.g from 100 liter "Oil I buy " you can produce 120 kg "Oil for use"  .

You would attach a Nomenclature (Bill of materials) to the product to be produced

Odoo doesn't allow a single product to have multiple units of measure from different categories, so you can't mix weight and volume.  I think the recipe would need to be converted to use litres of oil.