Can you mix synthetic oils with regular oil?

Yes, they even sell it that way.

Yes it will not.cause harm but you , might want to check the blend . Meaning that you may want to see if you are doing a 50/50 blend Ect

In order to ensure if you can mix these two syn oil, you should carefully check the base of each oil (cross check the oil formular). If you find they have same oil-base, you can mix it

Yes, according to Valvoline's web pages they are compatible. There are however some instances where fully sythetic oils should not be used, e.g rotary engines and wet clutches. So it is advisable to...

Note: If you can't do this safely take it to a shop. If you hurt yourself it will be your fault. Not recommended on any of the post 1999 VW Passats but: Run engine till warm, shut down. lift vehicle,...