Cessna Excel flight crew received LO HYD LEVEL annunciator after takeoff. They continue to their destination where a distraction caused by an FMC problem resulted in an overshoot of their cleared descend to altitude.

Narrative

After takeoff the LO HYD LEVEL annunciator illuminated. The appropriate checklist was run and the company was contacted for instructions; whether to continue to our destination or return to the departure airport. It was decided to continue due to long runways and better maintenance facilities (checklist list called for landing as soon as practical). Enroute all checklists were reviewed associated with a LO HYD LEVEL annunciator (i.e. no flap landing and manual gear extension). A crossing restriction was given by ATC. During the briefing of the possible issue; the aircraft suddenly started an un-commanded turn to the right. The pilot flying used the heading mode and corrected back towards the assigned course during which ATC queried us as to what our heading was. This led the pilot flying to disconnect the autopilot and increase the rate of bank and descent rate to comply with the assigned crossing restriction. As pilot not flying I was heads down operating FMS to put us back on course. I glanced up to check our position and noticed we were descending through 13000. I made the call to the pilot flying and he stated he was correcting. The altitude deviation was approximately 200 FT. It is unknown to both crewmembers as to why the FMS commanded the turn. The pilot flying stated he was too involved with solving the FMS issues as well as the LO HYD LEVEL problem which distracted him from the basic scan. The pilot monitoring was heads down solving the FMS issue and did not keep a frequent scan of the aircraft. It is important to maintain the basic concept of a crew and the division of duties.