We have a 2001.5 1.8L Turbo VW Passat with 65K miles on it.

I think the dealer used synthetic oil this oil change for the first time.

Once you use synthetic oil, can you switch back to regular? If not  
recommended, what happens if you do it anyway? (Just wondering)

I understand the 2001.5 VW Passat came with regular oil but can use  
synthetic. Is it better to use synthetic? Why? How much better?

Normally, VW changes our oil every 5K miles. Is it the same schedule with  
synthetic?

Can a mechanic tell what type of oil is in the car?

I needed to add some oil two months ago and called the dealer to find what  
kind I should add. He told me, since it was a 2001 turbo that it takes  
synthetic oil so I added about half a pint of synthetic. Now I’m thinking  
I had regular oil in there. Any problems mixing a little synthetic with  
non?

Thanks very much!

Joy