Lubrication Consulting / 5 min
Switching to an Equivalent Oil: Compatibility, Flushing and Compliance
Even after finding an exact technical equivalent for your current oil, draining the old oil and pouring the new one straight into the system often carries serious risks. Two different oils mixing can lead to chemical reactions, foaming or clogged filters.
Published by: Rotek Technical Content Team
Content is published based on manufacturer technical documentation and equipment requirements as the basis for final selection. This is general guidance and does not replace equipment-specific manufacturer approval.
Review our editorial guidelinesTable of Contents
- - Compatibility Between Oils
- - Base Oil Incompatibility Risks
- - Seal and Elastomer Compatibility
- - System Flushing Procedure
Compatibility Between Oils
Anti-wear (AW), anti-foam or anti-rust additive packages used in different brands' oils can be incompatible with one another. When two incompatible oils mix, the additives can react and precipitate (turn to sludge), clog filters, and completely destroy the oil's air-release properties.
Base Oil Incompatibility Risks
Mineral oils and PAO synthetic oils are generally compatible. However, a PAG (Polyalkylene Glycol) based synthetic oil should NEVER be mixed with a mineral oil. When they mix, they form a thick, gel-like sludge at the bottom of the system and can stop the machine entirely.
Seal and Elastomer Compatibility
When switching to a new technology or base oil, the response of the machine's sealing elements (o-rings, gaskets) matters. Certain synthetic oils (such as esters) can swell or shrink specific rubber types. The machine manufacturer's approval of the sealing material should be obtained before making the switch.
System Flushing Procedure
Even after draining, 10-15% of the old oil remains at the bottom of the system, in the piping and in the pump. For a safe transition, the system should first be run and cleaned with a special low-viscosity flushing oil, filters should be replaced, and only then should the new oil be added. Remember that a risk-free, definitive suitability decision requires OEM approval or an expert field assessment. Send us your current product's brand, code and label information so our technical team can evaluate a safe transition.
Frequently Asked Questions
Is flushing mandatory during an oil change?
If switching to a completely different base oil type (for example, from mineral to PAG synthetic), or if the old oil has severely oxidized (burnt) and formed sludge, system flushing is absolutely mandatory.
How is an equivalent transition done for greases?
Switching greases is far riskier than switching liquid oils, because different soap structures (for example lithium and polyurea) combining can cause the grease to suddenly liquefy and run out of the bearing. Never switch to a new grease without fully cleaning the bearing first.
Can different-brand but same-viscosity hydraulic oils be mixed?
While there's theoretically less risk when hydraulic oils share the same international standard (for example DIN 51524 HLP), differing additive chemistries still create a foaming risk. The safest approach is to drain the system fully and continue with a single brand's product.
How is a compatibility test performed?
To test the compatibility of two oils, the old and new oil are mixed in specific ratios (for example 50/50, 10/90) in a lab, heated and left to sit for several days, checking against scientific standards (ASTM D7155) for precipitation, cloudiness or foaming.