Hermetic Sealed Transformer

No, unless you suspect a problem, or the life cycle of the oil has expired.

What do the transformer specifications and operators maintenance manual say?

Why do you ask?

Why was your reply marked off-topic? Your questions are perfectly natural and reasonable questions to ask in light of the OP's post.

To the OP: Well? What do those docs say and why do you ask?

Somebody didn't like it?

I always like your answers Lyn! Sometimes it's the only entertainment I get in a day, and very often enlightening!

You want entertainment?

Do what I do: start trouble.

I reckon so. Gave you a GA to wipe the slate.

Not only should the oil be analyzed for dissolved gases, but also do aa general oil quality screen. Testing for dissolved moisture will point out if your hermetically sealed unit is not actually sealed as well as you think (sprung a leak somewhere!). Tests such as interfacial tension, acidity, specific gravity, and color number will point out deterioration of the oil due to oxidation, impurities, overheating, and other factors. The cost of the tests is cheap, and the information you get from them can help you head off a costly transformer failure by pointing out repairs or corrections that will not cost you nearly as much as a new unit.

I thought the idea of having a hermetically sealed transformer was to avoid contamination of the oil.

I think the oil has a X hours rating, and this is long enough to last the life of the transformer. Unless the transformer has been run at well above its rated capacity for a long time, the oil should remain "good".