Since the VW Passat 1.8L engine has an Inherent Oil Sludge Problem, I would run an engine flush thru it.
Auto-RX I have heard nothing but good things about this product.
Once you get to the Rinse phase of the Auto-RX I would suggest using Castrol GTX 5W�30 and a Quality Wix Filter #51393.
Then once you have completed the Second Application and Step 5.
I would switch to a Full Synthetic designed to meet the VW Oil Standard 502.00 like AMSOIL's European Car Formula Synthetic 5W-40 Motor Oil (AFL) and a Quality Wix Filter #51393.
Models with 1.8L Engine codes AEB, ATW, AUG, AWM only use engine oil that conform to volkswagen oil standard VW 502.00 (with viscosities of SAE 5W-40, 5W-30 or 0W-40).
I would also on every other oil change use the Engine Flush Available from Amsoil. I would also keep a very detailed Maintenance log and all receipts.
I called VW the MAX oil change interval for the 1.8L engine is 5000 miles.