Why my car smoke blue at startup ?

Sounds like valve seals, guides. You can probably drive like this for a long, long ways. If it ever gets to the point that oil consumption is a worry, you could change to regular oil.

Is it only on start-up, and the smoke goes away after a short while? That s worn valve guides/seals. It's because oil leaks down into the cylinders while the engine is off, and is burned when the engine fires. It's normally only obvious when the engine has been off for quite a while, such as overnight.

It'll probably happily go on like this for quite a while.

Worn valve guide seals and possibly worn valve guides.

You can leave it like this. Oil burning is not a good thing but as long as you keep up the maintenance it should not limit the car's life.

Burning oil.

If only at start-up my first guess would be worn valve stem seals.

If so, it's not critical but the problem won't go away on it's own. However, it could get worse

bad seals or gasket.

why you no english good

Dark smoke indicates there is some oil getting into the burn chamber. FWIW, white smoke indicates water is getting into the burn chamber. On the low (cost) end, it could be in need of a head gasket or worn valves. A cracked head or block would be more expensive to repair.

good question!