No need to change the oil. The Pennzoil conventional will perform fine over the rinse phase interval. Just remember to top off the lost oil in the filter change.
It is quite possible that your 1.8t was not sludge up to bad at all. I have no idea what your oil change intervals and oil choice over the years. Not every 1.8t motor sludges up. The ones well cared for do quite well. VW did an injustice to some folks by recommending 7500 mile oil change intervals. It would be one thing if you drove 7500 miles in 6 months. But if it takes over a year to drive 7500 miles that would another story. A bunch depend on how many miles are driven typically each time the car is run. For example if you drive the car 30 miles, 5 days per week on the way to work and back, this is much different than someone who starts the car and drives only 1 or two miles. For the car that only sees a short run with each start, a large amount of condensation will build into the oiling system. At the same time the computer makes the motor run a rich fuel air mixture, which allows for a lot of unburned fuel to pass by the piston rings during warm up. Between unburned fuel and moisture trapped in the cavity of the motor provides for the perfect catalyst for sludge formation. If you are someone that drives the car for several, 10 to 30 miles with each start, then the motor has time to build internal heat to the point where the moisture and unburned fuel gets eradicated out of the oil system via the PCV valve. So 7500 mile on an oil change is not a one size fits all. It all depends on your driving habits.
The other factor that must be considered is the effect on oil life due to the turbo charger. As you likely know, the turbo uses extremely hot exhaust flow to drive an impellor that is designed to force air down the throat of the intake system. A great concept to create a lot of performance out of a very small displacement motor. But also remember that there is an oil line that feeds lubricant to cool and lubricate the turbo bearings. The oil that flows to the turbo really gets super heated. At the same time with the turbo boost there is a much higher level of combustion gases slipping past the piston rings, some of which is unburned fuel. Therefore I think you can see that this combination is tough on motor oil to start. The advantage is that you have a motor that delivers economy due to it size, yet can produce much higher performance when you want it. One of the biggest problems that 1.8t owners suffer is what is commonly called turbo turds. This is the turbo bearings rolling up balls of oxidized oil that get fed back to the oil pan. The balls get caught up in the mesh screen of the oil pick up tube. Unfortunately the balls are not small enough to pass through the screen, where the filter would catch them, but are not large enough whereby they wouldnt lodge in the screen or mesh. The reason these balls form is that when you shut off your motor, all the heat is still present at the turbo and maintained from the catalystic convertor. But now you have no oil circulation. The oil left in the turbo unit and immediate lines is subjected to high heat for a pretty long period 0 f time. Spent motor oil will gell up and when the turbo starts spinning these balls and off they go back through the oil lines to the oil pan where they get caught in the screen.
After your Auto-Rx application, you should return to full synthetic motor oil. And I would suggest that you minimize your oil change intervals based on driving habits. We also suggest that you read the auto-rx maintenance dose instructions. What this boils down to is adding ¼ of a bottle of Auto-Rx or 3 fluid ounces with each oil change to not only maintain the gains from your cleaning treatment but also continue to clean and keep the motors internals clean. The maintenance dose has proven to be very effective in your turbo motor as well as Volvo and Saab units.
