Normally How Long Could a Jack Support with Oil Inside?

Hold what? I would expect a quality jack to hold all the reservoir oil for a half year. I would not be surprised if a cheap jack failed anywhere two weeks after anything. I would expect this Jack to hold a smug face through anything.

hold between 2 concrete structures under ~400t.

Depends on the leakage rate; if zero, yes. That might be uncommon, however.

Forever. And beyond.

If the jack is on it's side.

Use the jack to install adequate support that will last for 'more than half a year'.

Why are you asking a poor jack to do, what it is not supposed/made to do? Please do not kill jack.

Out here in hillbilly land, we just use the jack temporarily, then we use concrete blocks. They last forever.

What a waste and what a risk. Stick a large lump of something incompressible into the gap, then remove the jack and do something else with it. Sheesh.

If he wants it jacked up for 6 month stretches, I'm guessing it's a Cavalier.

The best option there would be to stick another Cavalier underneath it, then.

GA from me

until jack got tired and left for lunch

Excellent point!!!

I got caught up in the incredibly minimal amount of information that I overlooked that the tiny information implied a hazardous practice.

I don't know how anyone could possibly draw any conclusions at all about what the OP is doing, based on the complete lack of information provided.

The two key words: "jack" and "support".

I have no idea what the OP is doing either. One possibility is dangerously wrong and has now been indicated as such.

Right. You and gringogreg have both given good advice.

It would be nice to know what OP's up to.

a single piston jack? a floor jack? what type of seals does it have? how often has the fluid(or oil as you call it) been serviced?

It doesn't matter. Jacks are for lifting, not for supporting.

today I learned more about these little devils than I ever expected to.it turns out these things actually are tested and rated....something akin to a UL rating but for jacks by

NSI Standards...and they say how long it holds is one of their tests. I couldn't care less but I learned something today from CR4

"You couldn't care less" but would you be willing to put your head under anything that it took a jack to lift, regardless of what the jack was rated for?

We never heard back from the original poster. Perhaps he is stuck under whatever he was trying to support with his hydraulic jack and is not able to reply

If I seem obsessive about this subject, it is because of the 15 years I spent in the fire service, and the training that I received in the safe use of lifting devices. I was, in that time, involved in the rescue of a number of people who were trapped under heavy objects, some due to the misuse of lifting equipment.

Its okay boys, he is gone, and we will never hear from him again.

Personally, I can't imagine wasting a moment on somebody who can't be bothered to register. (or sign their name)

I would agree, except that people other than the OP might learn something from our replies. This is the main reason that I follow this site. I have learned a tremendous amount from the replies by those who have expertise in areas that I am not experienced in