Volkswagen Passat Engine Problems
Volkswagen Passat 2006: Oil pressure dropping after top up?
It’s not related to the oil, the oil you used was correct. It shouldn’t lose oil pressure so quickly, if it does there is something going on within the engine that needs addressing. If the engine is making a bad sound, don’t drive it, and get a mechanic to check it.
Which Passat is better, diesel or petrol?
Both are fine choices. I believe it comes down to the use you plan for it. A diesel works at its best when it's constantly running and fully warmed-up, preferably at a constant speed. The petrol engine is better in transient traffic conditions where the car is always accelerating or slowing. If you live in the city I would go for the petrol engine, if you live out of town then the diesel is probably better.
Passat using oil?
I would continue to use he same oil you've been using, I doubt that needs anything else and I wouldn't put a "thicker" oil in it. VW engines have a habit of using a little oil, enough to make it necessary to keep a constant watch on the oil level and keep it topped up, but it has only done 175,000km and is only using oil on the highway.
Passat performance chip
I would urge caution. Chipmakers extract greater performance from cars by going closer to the edge of destruction than a manufacturer would dare do. Do your homework and establish what the chip will do to the calibration of your car and assess the possible problems that could occur, and then make your decision.
VW Passat engine
Don't listen to "European car motor mechanics" for starters. I reckon if you checked in his back yard you'd find it full of Passat engines he would like to sell you. It's not a good idea to only do very short runs where the engine, gearbox and differential don't get a chance to warm-up, but runs of 10 to 50 km are fine.
Cam belt or chain
It has a chain and doesn't require servicing.