Can gearbox oils be mixed together?

There are four main families of gearbox oils:

  • Mineral oils 
  • Poly-alpha-olefins (PAOs)
  • Esters 
  • Polyglycols, also referred to as Poly Alkylene Glycols (PG or PAG)

Mineral oils, PAOs and esters may all be mixed together.
Only polyglycol type synthetic oils (PG or PAG) may not be mixed with the other lubricant types.  They can react together and form gum gels in the casings. Furthermore, they can sometimes be incompatible and immiscible with other polyglycols.  

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