We have analyzed a hydraulic oil (Shell Tellus T 15) and have found higher Phosphorus (And Si) than excpected in this oil. The Phosphorus content is app 3000 ppm (Normal is 4-500 ppm).

Shell claims it most likely is an additive that contains: 6-6.6 % phosphorus, 11,4-12,4 % sulfur and 7,6 - 8,1 % zinc. Treat rate 0,84 %.

We do also have app. 150-200 ppm of Si in this oil (oil is used for flushing new hydraulic systems). Microscopy shows few particles (non dust like) so the Si is most likely also from a product.

Does anybody know what substance this might be?

Silicone based defoamer.

did U analysis oil sample for Mn & Cd
it couled insulbel toluen

Most Mineral based oils will have a silicon based additive, but at the levels you are talking it would be an ingress of some sort.
What type of seals are you using in your system ?
What type of breather arrangement is used on the Hys. system, ie silicon based gel.