I recently bought a used VW Passat diesel 1.9 TDI (1997) model. It used to burn oil so I send it for general maintenance in the motor (pistons springs, valves, turbo). It seems that after the first 600 km burned about 250 ml of oil. Is this normal. I read in many forums arguing that this is normal or others to say that it should not happen. So my question is if I can prevent this burn or not. Should I worry or just add oil every now and then?
If your engine is burning 250ml of oil in merely 600km, It'd be EXTREMELY worried, and think that'd be an oil leak rather than literally burning oil. And, yes. In a diesel particularly, you need to be careful with oil. If your engine drinks too much as a diesel, it's going to perform a 'diesel runaway.' If you don't know what that is, it's basically the diesel engine using the oil as fuel. Because a diesel works off of compression rather than spark plugs to burn the fuel, you can't so simply turn off a diesel. You'd have to shut off the fuel pump. If it's getting fuel from another source like engine oil, though, you can't turn off any feeds. That means that even with the key half-way around the world from it, it'll still be running if it's got enough oil that will flow indefinitely.
I would take the car back to whoever you got it 'fixed' by and complain, maybe even make them fix it properly for free, because this is a VERY bad situation. Once it has a diesel run-away, you will need a new engine unless you block the air intake in time.
You can prevent the burning by getting it fixed properly. If the seals aren't placed in properly or if the piston rings are still worn, you're going to need to get it fixed or replaced.
Good luck, and I seriously think you need to get it sorted properly.
You don't really give us much detail of what was done to your engine when it was rebuilt. I have rebuilt quite a few VW diesel engines, and not experienced what you describe. High oil consumption generally comes from two areas: the valves or the cylinders. Sure, there are other sources like a leaking head gasket.
If your cylinders are leaking, compression test and a leak down test would be helpful diagnosing the problem, and whether it is one cylinder or more.
Were you given any break-in instructions?
Normally I expect higher oil consumption, until the rings "seat" and do a better job of controlling oil. For my diesels, both IDI and TDI, which I have re-ringed (never needed pistons), I would not idle the car much, but rather drive it fairly hard, and try to avoid extended times of lower power settings.
I would consider a two pronged approach: 1. Review breakin instructions, and 2. compression check and leakdown check the engine on each cylinder.