What must be known about the compatibility between different lubricants?

Before any change or complement of lubricant in machinery, it is essential to ensure their mutual compatibility. 
A compatibility test carried out in a laboratory might be necessary.
Some well known cases showing compatibility issues:
- Synthetic oils based on polyalkylene glycol (PAG) are incompatible with mineral oils and other synthetic oils. Oils formulated based on PAG are also not all mutually compatible.
- Ash-forming oils, for example oils containing zinc dithiophosphate additives, are generally not compatible with ashless oils.
- Replacement of a fluid grease with an oil.
- Replacement of a grease by another incompatible grease.