some questions about oil change
 

I am driving a 2011 Volkswagen Golf.
And I have some questions about oil change.

1. On my owner's manual, it says that in order to get a true reading, the engine must be at operating temperature. What does it mean by operating temperature? Does it mean I have to turn on the engine for like 5 mins then switch it off? Or more longer than that?

2. On the owner's manual, it says that the engine was factory filled with high quality synthetic all season engine oil that meets strict volkswagen oil quality standards and have a viscosity grade of SAE 5W-40.
Then I found this engine oil by Mobil
http://www.mobiloil.com/USA-English/...l_1_0W-40.aspx
On the specification tab, it says it is approved by VW 502 00 and 505 00 (ok, quality standards checked)
HOWEVER, it is not SAE 5W-40, it is 0W-40, can i still use it?

3. Can i mix different engine oil?
if it is a yes to Q2, can i add that egine oil from Mobil with the unknown oil added from my dealer?

ThankS!!!