Can I Substitute Hydraulic Oil for Transformer Oil?

If you are asking this question, you are likely dealing with a transformer right now – perhaps trying to troubleshoot, add more oil, or are in some sort of situation in which transformer oil isn’t immediately at hand, perhaps an emergency.

So here is the straight, non-sense answer. Absolutely not. You should never use hydraulic oil in place of transformer oil – not even for a brief moment.

There are zero exceptions.

Let’s walk through why this matters, what could go wrong, and what you can safely do in the absence of transformer oil.

Why Is the Answer “No”?

Most people think oil is oil — but in reality, hydraulic oil and transformer oil do completely different jobs.

Transformer oil is designed for:

Electrical insulation
Heat dissipation
Preventing arcing
Protecting the cellulose insulation system

Hydraulic oil is designed for:

Pressure transmission
Lubrication
Wear protection (AW additives)
Mechanical system efficiency

The key point:

Hydraulic oil has almost no electrical insulating capability.
It will break down instantly inside a transformer.

Hydraulic Oil vs. Transformer Oil

Feature / Property	Hydraulic Oil	Transformer Oil
Primary Purpose	Pressure + lubrication	Electrical insulation + cooling
Insulation capability	Nearly zero	Extremely high
Additives	Extremely low moisture destroys insulation strength	Designed to be noncorrosive to insulation
Compatibility with paper insulation	Will degrade it	Fully compatible
Moisture tolerance	Medium	Extremely low — moisture destroys insulation strength
Breakdown voltage	Very low	Very high (critical for transformer safety)
Risk inside a transformer	Immediate failure	Safe

What If I Already Used Hydraulic Oil?

This is the part where most people start to feel anxious, as well as to be honest, they should. When hydraulic oil is in the transformer, what should you expect:

Insulation failure; hydraulic oil breaks down under voltage, then instant dielectric collapse occurs.
Short circuit in the winding: once insulation fails, the winding is exposed.
Overheating, arcs, and possibly explosions: Negative failure is the only thing transformers do; they certainly do not fail gently.
Fire hazard: Hot oil combined with insulation and electrical arcing poses a big hazard.
Insulation paper is damaged: AW/EP additives attack the cellulose, and this is not reversible.

In simple terms, putting hydraulic oil in a transformer is the same as if you used soda as brake fluid.

In this scenario, you will be driving a car that is already broken.

I Don’t Have Transformer Oil — What Can I Do Safely?

Good news: There are safe ways to handle the situation without destroying your transformer.

Here’s the correct approach:

Stop the transformer and wait for the correct oil

Stopping the unit is always safer than adding the wrong oil.

Running without enough oil is safer than running with the wrong oil.

Use a compatible insulating oil (NOT hydraulic oil)

If mineral transformer oil is unavailable, you can use:

Natural ester insulating oil (vegetable-based)
Synthetic ester insulating oil
Silicone insulating oil

But — and this is important: Only if your transformer is rated or retrofittable for these fluids.

If hydraulic oil has already been added

Please complete the following steps in case of emergency:

Please shut down the transformer immediately.
All the oil needs to be drained.
The tank and the windings need to be vacuum filtered at least 2 to 3 times.
Let the breakdown voltage be tested.
The insulation needs to be checked for moisture, and the dielectric dissipating factor needs to be checked.
DGA (Dissolved Gas Analysis) needs to be performed.
The insulation paper needs to be checked.

The transformer oil needs to be emptied, and the correct oil needs to be added.

Summary

Can you substitute hydraulic oil for transformer oil? Absolutely NOT.Not for a minute. Not in an emergency. Not “just to test.”Zero safe scenarios.

Why not?

Because hydraulic oil:

has almost no dielectric strength
contains corrosive additives
destroys transformer insulation
can cause short circuits, fire, or explosion

What to do instead?

Stop the unit
Use proper insulating oil
Follow strict cleaning procedures if contamination has already happened

Final

Transformer oil exists for a reason — use the right one.
Hydraulic oil will 100% destroy a transformer.