How do you switch from synthetic to regular oil?

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, remove belly pan

remove oil filter.

remove drain plug and drain until one drop every 15 seconds exits the crankcase.

Install drain plug, make sure the copper plug seal is in good shape, lubricate gasket on filter with clean oil, install tight, 3/4 turn after gasket contact on the engine adapter.

Install new oil, 5 qts typically.

Start engine, do NOT rev, keep an eye on the oil light, let the oil pressure rise and stabilize. Check for leaks.

replace belly pan.

Change oil MORE often , every 3000 miles or less under severe conditions.

I agree with all of the above except the interval. It is a waste of oil and money to change your oil every 3,000 miles. That was good advice in 1955 but not in 2010. Modern engines with modern oil and filters can be safely changed every 5,000 miles even in severe conditions. You can go up to 7,000 in normal conditions and 10,000 if you are using synthetic oil.