Question: Can I use 15w40 synthetic oil in my 2015 vw jetta tdi and how long can go between oil changes?

Answer: 
The same belt is possibly driving the power steering pump and the alternator. If the belt is slipping when you first start the engine, then the power steering will not work correctly and the alternator will not be providing power.

Friction on the slipping belt may then make it heat up and start to grip. The power steering would then start operating and the alternator start producing power. The increase in voltage when the alternator starts working would make the AC fans run faster.

You may need to re-tension the belt, or replace it. 

If this car has an electric power steering pump, then alternator belt could still be slipping and would therefore not be able to supply enough power to the electric pump and the AC fans until the belt stared to grip.

Either way, I would check the belt before replacing the battery.
For my 2000 toyota sienna (which has belt+fluid driven power steering) doing a complete fluid flush helped (turn the wheel back and forth 10x afterward to flush out air bubbles), FWIW...also probably worth making sure fluid isn't low first...

After that it's replace hoses, then replace pump, I guess.

ref: <https://answers.yahoo.com/question/index?qid=20090412100118AA3kc5m>