What risks are there in using a lubricant that has not been approved by the manufacturer?

As a general rule, a gearbox lubricant meets the requirements of an international standard, such as ISO or DIN (For example, ISO CKC or CKD). A manufacturer references or approves a lubricant in relation to these international standards, but also on the basis of its own criteria, such as compatibility with the paints that they use or the seals. From a mechanical perspective, the risks are practically zero if the product is in compliance with the international standard in question, and the standard required by the gearbox manufacturer. The need for a product to be referenced or approved is mainly to do with security issues during the warranty period for the manufacturer.