I'm guessing its a diesel, not the v8. It its worn that should become apparent almost as soon as it warms up and you will get oil loss and smoke from the exhaust. I wouldn't change to a thicker oil unless it really is worn. The Tdi diesel is a robust unit if its looked after, plenty do 300,000 plus with no trouble, mine has 205,000 on it and doesn't burn a drop.
Blocked fuel filter or tank breather sounds more like it. If you stop and take the filler cap off after a run, is it hard to shift at first, then you get a rush of air into the tank
You could try asking on http://www.difflock.com – helpful bunch on there