Hard to tell. I did notice a little more engine noise than with heavier oil. I sent in a sample of the Mobil for analysis, and they said it was pretty much worn out (additives depleted and viscosity dropping) at 5,000 miles. I switched over to (what's sold in the US as) Shell Rotella T6 5w40 heavy-duty engine oil. It's a synthetic marketed toward big trucks. After 5,000 miles I sent in a sample, and it was in much better shape-they recommended trying a 7,500 mile interval. It's less expensive than the Mobil to boot. I don't think it's on the VW/Audi list of 502.xxx approved oils, though, so it's probably not appropriate for use during the warranty period.
I'm envious of your TDi, I've thought about trying to get an engine and transmission sent over for a conversion. Even a 6th gear from a TDi seems like it would help, but they want $900 for a pair of gears here.
