There are nearly as many additives as there are opinions. PowerService is ok. So is Red Line's DFC and probably many others. My favorite when it comes to quieting diesel knock is a mix of Amsoil's Cetane Boost and their Diesel Additive Concentrate (2 separate bottles). The main things to look for are does the additive increase lubricity, detergency and cetane, and does it disperse water? Those are the most important things. An additive that does all of these things is better than one that does only some of these things.

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2004 VW Jetta TDI (manual)

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