thinking switch back to regular oil

well I been using mobil 1 syntec 10w/30 for 3 year now . When first time I bought my car it already had 103k mile , now it had 118k ( it been 3 year ). Any way after a month using syntec , I had oil leak ( it leak from the seal between the engine and the tranny ) , back in the day I dont mind fill up a quart every weak , but now the car had little smoke in the back , and I just take it to the mechanic to check the price ( it cost $600 to fix the leak ). He say I should switch back to regular oil and using the stop oil leak fluid ( u know those " barleak ). So now im thinking switch back to regular oil , but dont know what oil to chose . exp : max life , castrol gtx , or mobil 5000 ( I guess this one not synstec ). Any common ? thanks ( please trying to understand my english ).

I don't suggest you switch back, unless it's to a synthetic blend. Your car is used to synthetic now, it will do some weird shit if you switch back after using it for 3 years.

do any one know any tip or and fluid that help stop leak ?

It cleaned all the gunk from your motor. Sounds like your rear main seal is leaking and maybe your valve stem seals is where the smoke is coming from. Both will be some pretty major work. I'm not sure about any type of stop leak since I have never used any. If you switch back it will still leak from the same spots. You just might have to replace all the seals.