For Syria you dont need a carnet, but youll have to buy some import sheets for the non-permanent import which are round about44$ or 48 $ road tax 6 or 8 $ + insurance which is another forty something (46 or 48 $ valid for a month) ( 100 $ diesel tax per week if you`re travelling with a diesel car)
For Kilis which I used 7 or 8 times I ve never met long queues of buses and trucks; Kilis is very efficient,correct and friendly; after the gate the bank is on the right hand, all the car stuff is in the middle and police with passports is opposite on the left side. When you enter with a diesel car dont waste the first day entering Syria in the evening, because the day you enter counts as the first day of those 7 mentioned above-- if you overstay those 7 diesel days lets say only for another day youll have to buy another week which is again 100 $ or you`ll have to bargain hard, but may be not successfully.
Since fuel is very much cheaper in Syria, dont waste your money in Turkey, but have some kilometers left; the next fuel station at the Kilis border is 4 to 5 kilometers after the border, but sometimes theres no diesel. Safe for fuel is Aleppo.
If you take the Reyhanli - Bab al Hawa border add to the above some 10 to 15 € for "helpers" (obviously some kind of contractors with the border staff) plus one or two hours for getting helped.
Dont know where you leave Syria, but obviously it is a must that you go back to Turkey - cause for Jordan you need a carnet, same thing with Lebanon; Lebanon is also nearly impossible to enter with a diesel car for environmental reasons (except if you have a very good story and some green bucks to make communication easier and even with that its not sure that you can enter )