You guys are not looking at all the issues. The first number (before the W) is the cold viscosity. You want that to be fairly low. The second number is the hot viscosity, you want that to be fairly high. A 5W-40 oil will have almost the same viscosity at operating temperature as a 15W-40 oil, but it will flow better at startup which is what you want. Cold starts are probably the #1 source of wear in most engines. That's why I also like to add some Lucas oil stabilizer to my synthetic 5W-40 which I believe further helps reduce wear on cold startups by making the oil a little more sticky. Now if you're talking about 0W-20 or 5W-20 oils, yeah those are bad for the engine though they increase the fuel economy a tiny bit. A 20W oil will flow like water at operating temperatures.

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2004 VW Jetta TDI (manual)

Past MB's: '96 E300D, '83 240D, '82 300D, '87 300D, '87 420SEL