I'm using Titan HD Super SAE40(German oil) since i got my car from 3 years,which is recomended by the mercedes dealer in Kuwait.I tried once Elf semi-syntethic(10-40)which is french oil and it gave better performance and smoother ride but my engine starts to leak(down the oil pan).Tomorrow I'm going to change my head gasket and some other gaskets and seals.So is it safe to use Syntethic oil after i change my gaskets or still it will leak???

absolutely

as someone told me about my diesel chevy truck, engines with quite a few miles tend to leak a bit more with synthetic oil because it is slicker and tends to slip through more easily than dino oil does.