Oil Consumption: Causes and Solutions

Could you please quickly tell me the cause of the two 'effects' you're referring to?
1. You let the car roll downhill in a higher gear (in drive mode), and after a while, you accelerate, and a huge cloud of oil appears behind you.

2. The car is constantly emitting smoke, even when idling.

In both cases, the oil level is correct.

Based on the mention of a "sliding" noise, I would suspect faulty valve stem seals.

Okay, so for the first one, replace the valve stem seal.
and for the second one, are we talking about oil wiping rings? or both?
Okay, yes, the oil consumption can be up to 2 liters for every 1000 kilometers.

A combination of both errors cannot be completely ruled out. However, replacing the valve stem seals is much easier, so I would start with that. If you have compressed air available, it should work quite well with the head still installed, as far as I know.

Switching to a slightly thicker oil might help reduce oil consumption until the final repair is completed. I definitely wouldn't use a 0W-30 oil in that type of engine. Instead, something like a (approved) 15W-40 or 15W-50 would be more appropriate.

It certainly sounds like a valve stem seal issue, but just to be sure, I would check the following points first:
Measuring the internal crankcase pressure: Alternatively, with the engine running, unscrew the oil filler cap and observe what comes out and at what pressure.
If everything is alright, it's probably the valve stem seals.
Wenn nicht: KGH prüfen. Wenn verstopft, reparieren, ansonsten sollte der Fehler im Bereich von Zylinder u. Pistons are lying.
What kind of engine do you have?

Thank you for the answers.
It's just about old Opel gasoline engines (C14NZ), that's why I didn't mention it, as it doesn't really belong here, but where else on the internet can you find so much specialized knowledge?
'Yeah, I wouldn't put 0W30 in there unless I got it for free. Over the course of the year, I've used 20W50 mineral oil, 15W50 full synthetic, 15W50 mineral oil, 10W40 semi-synthetic, and 15W40 cheap oil – basically, anything I had lying around. The only thing I noticed with the thicker oils was a slightly reduced engine performance, if anyone's interested.'

Avenga wrote:

These are simply old Opel gasoline engines (C14NZ),

This makes valve stem seals all the more likely to be the cause.
You should definitely write about this right away. While there's a section for non-TDI models, it probably won't be as popular overall.

These are simply old Opel gasoline engines (C14NZ),
This makes valve stem seals even more likely.

absolutely

'My beloved 1.3S had worn piston rings, while the C20NE, despite having 200,000 km on the clock, ran like a Swiss watch...'

CU Gremlin.