oil blacked and generate particlles

Make sure the oil is compatable with the OEM's requirements. Using the wrong type of oil could cause oil seals or o-rings to deteriorate, a possible cause of the sedimentation you are seeing. Of course, you would notice alot of leaks and possible operating problems if this were the case.
Is the oil sump bottom clean? Dirt can be circulating from a dirty sump. Do you have tank vent filters installed on your sump? The sump will breathe during operation and draw in dirt and dust from the outside air.
I suspect you just need a good system flush with clean filtered oil AFTER the sump is cleaned and wiped out. This process can take some time to do. I use a portable filter unit with a 3 micron element to empty the sump into a drum with a clean plastic liner installed. After the sump is empty, reverse your hoses and pump back into the sump. When full, start the machine and work all the controls to circulate the clean oil and flush any old oil back to the sump. Shut down and repeat the process three or four more times until the oil in the sump is clean and brite. Perform a particle count looking for an ISO 17/15/12.
This is what works for me and is quicker than just recircing the sump.

Good Luck!

ALDMOKY,

Hal has provided some good practical advice. C.C.JENSEN provides kidney loop filtration systems for exactly this type of problem. The advantage of flushing is in addition to cleaning the oil the internal assembly will be cleaned as well. Simply doing a normal oil change will leave behind much of the dirt. Have a look at these case studies where hydraulic oil is filtered to be even cleaner than new oil.

Komatsu 930 Hydraulic Oil Flushing
http://cleanoilbrightideas.wor...raulic-oil-flushing/

Drill Hydraulic Oil Flushing
http://cleanoilbrightideas.wor...n-atlas-copco-dm-m3/

The after filtration ISO for the drill was 10/8/5. For the Komatsu 930 Hydraulic oil was 15/12/7 after filtration and flushing.

ALDMOKY,

I am not so sure if the oil black (assuming oil color = 7-8) is just due to dirt/sand.
you mentioned that you found sediment in the bottom of the bottles. Just check wheather it is affected by magnet or not.
if it is affected, it may rubbing on the hydraulic pump.

possiblity 2, the operational temperature of the hydraulic system might high (> 83 degree C) which causing the oil become oxidized in 10 days. The sediment might just come from seal.

Do oil analysis may better to get the clear picture

Regards,