I can't give a great deal of detail about the actual dependence of Syria on Iranian oil. However, the situation in Syria vis-à-vis its economic environment as a consequence of sanctions is becoming extremely difficult. The currency is falling, and there is inflation. There are breadlines forming. There are electricity blackouts because of the lack of access to fuel to run their generators. The dependence on the oil is keeping that economy limping forward.
As for the threat to Israel, certainly Israel perceives the threat from Iran as an existential threat, a threat to its very existence. This is because of the pronouncements of the regime with respect to Israel's existence. The support it has provided to Hamas and Hezbollah has equally been a threat. Both Hamas and Hezbollah are showing themselves to be increasingly reluctant to be allied with the Syrian regime right now, and the Hamas leader has been rumoured to be trying to persuade Assad to accept the Arab League plan. Hezbollah has also said that it will not seek to engage in a confrontation with Israel at this time.
The threat is very real, but the players right now are taking some fairly cautious stands, which I think is indicative of the extent to which people are worried about the overall consequences of the instability that would emanate from Syria.