I have a '99 a4 1.8l TQM. My question is about the path of oil through the engine. Specifically filter and where the oil pressure switch sits in the flow, before the filter or after. I have been getting intermittent oil lights that started after I changed the oil using Royal Purple oil and their new filter.

 

The oil pressure sensor/sender sits right in the oil cooler right above the filter. If i had to guess it would be after the filter in the flow, but not positive.

 

I found an answer to part of my question. http://www.secondchancegarage.com/public/297.cfm
I hope this helps someone else too.