"Our customer mixed an 80W-90 gear oil into a storage tank that had roughly 5 to 10 liters of automatic transmission fluid still in the tank. Would this mixture compromise the oil to make it useless or could they still use this oil in their final drives and differentials in which they would normally use 80W-90 oil? I'm confident that it is only good for hinges now."

When different types of oils and greases are mixed together, it can spell disaster for machinery. This is why it’s best to avoid mixing lubricants. However, since you are dealing with small quantities being added to larger volumes of oil with similar grades and viscosities, this may not be as bad as it seems. Several factors will determine whether you have an expensive mistake or a close call that you can hopefully learn from and mitigate in the future. These key factors include the volume of the oils that have been mixed together, the types of base oils that have been mixed, the types of additives that were in the base oil, and how the viscosity has been affected.

The volume of the oils that have been mixed together will be key. For example, if you mixed 1 gallon of automatic transmission fluid (ATF) with 300 gallons of gear oil from the same mineral group, the dilution would hardly be noticeable. On the other hand, if the ratio was 50/50, the resulting mixture would not be much good for either application.

The base oil type (synthetic or mineral) is also critical. Mineral oils like Group I, II and III oils and polyalphaolefins (PAOs) are very similar chemically, so mixing the two should not cause a compatibility problem. Conversely, in most cases polyalkylene glycols (PAGs) and PAOs cannot be mixed, nor should PAGs and mineral oils.

Additive packages generally are very specific to the machinery in which they are used. Motor oils and ATFs are high in additive concentrations, while hydraulic and turbine oils have low additive concentrations. Keep in mind that certain additives like phosphorus can be harmful to brass or copper components in machinery such as worm gears. 

Viscosity is the most important quality of a lubricant, and any changes to the viscosity can have a detrimental impact on machinery. If the viscosity increases, it could cause the machinery to overheat (fluid friction). If the viscosity decreases, the lubricant may not have enough film strength (boundary friction) to protect the machinery, which can also result in overheating through metal-to-metal contact.

It is essential to understand all of these factors. If there are any doubts, have the viscosity and additive concentrations tested or simply throw out the mixture. It is probably not worth the health of the machinery.